A Phenomenon Resembling Early Superhumps in a New SU UMa-type Dwarf Nova with a 2 hr Orbital Period

We investigate K2BS5, an optical transient that we identified in Campaign 13 of the Kepler/K2 archives by the K2 Background Survey, and classify it as a new SU UMa-type dwarf nova. Using the light curve generated from Keplers long-cadence observation mode, we analyze the dwarf nova during quiescence and superoutburst. Following 20 days of quiescence at the start of the observation, the system entered a superoutburst lasting 12 days, after which it experienced at least one rebrightening. K2BS5 clearly meets the criteria for an SU UMa star, but at the peak of the superoutburst, it also shows double-wave oscillations consistent with the spectroscopic orbital period, a phenomenon that closely resembles early superhumps in WZ Sge stars. While we do not classify K2BS5 as a WZ Sge system, we discuss how this phenomenon could complicate efforts to use the suspected detection of early superhumps to distinguish SU UMa-type dwarf novae from the recently recognized class of long-orbital-period WZ Sge systems.


INTRODUCTION
Cataclysmic variables (CVs) are a classification of binary star systems consisting of a white dwarf (WD) primary paired most commonly with a red dwarf (RD) secondary.Mass transfer between the two stars occurs when the secondary overflows its Roche lobe, resulting in the formation of an accretion disk around the primary if the WD is not strongly magnetized (for reviews, see Warner 1995;Hellier 2001).Often the accretion disk is thermally unstable and subject to recurring outbursts on timescales ranging from days to many years, depending on the mass-transfer rate (Osaki 1996, and references therein).These systems are known as dwarf novae (DN; for a review, see Osaki 1996).
SU UMa systems are a subcategory of DN with typical orbital periods ≲ 2 h that are distinguished by the occasional "superoutbursts" they experience, which are outbursts of longer duration and greater amplitude in comparison to ordinary outbursts.During superout-bursts, SU UMa systems show the presence of superhumps, which are periodic oscillations slightly below the orbital frequency.Superhumps result from tidal instability in the accretion disk, excited when the outer disk expands to the 3:1 resonance with the orbital period of the binary (Whitehurst 1988;Osaki 1989).As seen in Kato et al. (2009) and Kato (2022a), the superoutburst displays three distinct phases of period evolution, designated as stages A, B, and C. Stage A appears first in the evolutionary progression, characterized by the longest superhump period and no discernible period derivative.Stage B is the middle segment with a positive period derivative, followed by stage C exhibiting a shorter and more stable period.
WZ Sge-type systems (reviewed by Kato 2015) are a subcategory of the SU UMa-type that generally show only superoutbursts whose recurrence times are significantly longer than those of SU UMa stars.Like the SU UMa systems, they exhibit long-duration superoutbursts, but they can often be distinguished by the The more sparse sampling shows evidence of few notable events apart from one superoutburst occurring in September 2019.There are no normal outbursts beyond superoutbursts, a hallmark of WZ Sge systems.The superoutburst observed by Kepler whose observation time is highlighted in red, is not visible because the system was near solar conjunction at the time.
presence of subsequent rebrightening events and lowamplitude oscillations in the earliest stages of the superoutburst.The rebrightenings are also referred to as echo outbursts.The double-wave feature is referred to in the literature as early superhumps (ESH), which have approximately the same period as the binary orbital period.ESH are thought to be the result of expansion of the outer accretion disk to a 2:1 resonant frequency with the orbital period, which is possible only for very small mass ratios.The subsequent transition from ESH into Stage A takes place as the outer disk at the 3:1 resonance radius becomes eccentric and undergoes apsidal precession (Kato & Osaki 2013a).
Observing these short-lived phenomena during superoutbursts has proven to be one of the many accomplishments of the Kepler spacecraft during its original and K2 missions, both of which provided continuous light curves of predetermined targets for an extended period of time (often months or longer).However, while previous analyses of Kepler data have focused on known CVs, the Kepler archives contain numerous background pixels that were not studied directly at the time of observation, providing an opportunity to search for previously unknown, interesting objects.Within those background pixels, the K2 Background Survey (K2BS; Ridden-Harper et al. 2020) has uncovered transients by systematically iden-tifying potential transients that are then reviewed manually.One of the early successes of the K2BS project was the discovery of the only superoutburst of a WZ Sge system observed by Kepler /K2 mission (Ridden-Harper et al. 2019).
Here we present a photometric and spectral analysis of K2BS5, a new SU UMa-type DN.

DATA
Unlike the serendipitous background sources that the K2BS project normally seeks to unearth, K2BS5 was targeted in K2 Campaign 13 as part of a program (G013086, P.I.Patricia Boyd) to observe candidate active galactic nuclei identified from a search of archival Xray sources.In addition to the Chandra detection that led to its inclusion in that K2 program, it is listed in the 2XSPS Catalog of Swift X-ray Telescope Point Sources and Data Release 8 of the XMM-Newton Serendipitous Source Catalog; its identifiers in these three catalogs are CXOJ043436.4+180243,2SXPS J043436.5+180243, and 3XMM J043436.6+180245,respectively.We refer to the object here as K2BS5 because it was the fifth transient discovered by the K2BS project, but it has also been detected by the All Sky Automated Survey for SuperNovae (ASAS-SN; Shappee et al. 2014;Kochanek et al. 2017)   two additional identifiers (Gaia19emm and AT 2019sgc) as a result of the detection by Gaia of an outburst in 2019 (Hodgkin et al. 2019).

K2 light curve
During K2, Kepler suffered from a 6 hr periodic drift, causing targets to shift across the detector and degrading the photometric precision.For well-isolated targets, one way of mitigating this problem is to select a sufficiently large photometric aperture that all of the target's flux is captured, regardless of the drift motion.Using the interactive inspection tool in lightkurve (Lightkurve Collaboration et al. 2018), we selected a custom extraction aperture from the target pixel file data to encompass the system's full range of motion over the course of the observation.Additionally, we used lightkurve to visually inspect the images of the source in order to confirm that its brightness variations were attributable to variations in the target (and not from spacecraft systematics or the passage of an asteroid through the photometric aperture).The data were obtained during the Kepler /K2 Campaign 13, running from 2017 March 8 until 2017 May 27, a duration of ≈ 2.5 months, at a 30 minute cadence.

LBT Spectrum
We obtained spectra of K2BS5 with the Multi-Object Dual Spectrograph (MODS; Pogge et al. 2012) on the Large Binocular Telescope (LBT).Nine individual spectra were obtained on 2020 February 29 (UT) under cloudy conditions.The final three spectra had the strongest signal, and these were averaged to create the final spectrum with a total exposure time of 900s.The dual grating mode for MODS was combined with a 0.8 arcsec slit to provide a spectral resolution of R = 1860 at H β .Seeing during the exposures varied between 1.1 and 1.5 arcsec.
The spectra were extracted and wavelength calibrated using argon and neon emission line arcs.The spectra were flux calibrated using the spectrophotometric standard star Feige 34 obtained on a clear night earlier in the run.

Krizmanich Photometry
We conducted additional ground-based observations of K2BS5 on four nights during the first week of March 2021 using the University of Notre Dame's 0.8m Sarah L. Krizmanich Telescope (SLKT).The time was corrected to Barycentric Julian Date (Eastman et al. 2010) with astropy (Astropy Collaboration et al. 2013).During each of the 2 hour long observations, we obtained unfiltered images using 30 second exposures.The typical signal to noise ratio per exposure was approximately 14.0.Drake et al. 2009), the All-Sky Automated Survey for Supernovae (ASAS-SN; Shappee et al. 2014;Kochanek et al. 2017), Asteroid Terrestrial-impact Last Alert System (ATLAS; Tonry et al. 2018), and Gaia.Perhaps the most striking property of these data is the absence of outbursts; only a single undisputable outburst is present (in 2019), although there is a possible second outburst near the beginning of 2006.Owing to K2BS5's proximity to the ecliptic, there are significant seasonal gaps that could conceal additional superoutbursts.Indeed, the K2 light curve, the baseline of which is indicated in Fig. 1, recorded a superoutburst in 2017 during one of those gaps.
The 2019 outburst lasted for ∼ 2 weeks and has the shape of a superoutburst.The ATLAS data reveal that when K2BS5 emerged from solar conjunction in 2019, it was ∼ 0.3 mag fainter than usual and remained so for the next two months, after which it entered a superoutburst.

K2 Light Curve and Power Spectrum
In Figure 2, we show the K2 light curve of K2BS5, the most notable features of which are the superoutburst and a subsequent rebrightening event.The superoutburst occurs after at least three weeks of quiescence and begins with a steep rise of over 3 mag near BKJD = 3022, where BKJD is the Barycentric Kepler Julian Date, defined as BJD-2454833).After reaching its peak brightness near BKJD = 3024, the light curve begins to experience a slow fade, and large-amplitude superhumps appear.Near BKJD = 3033, the rate of fading increases dramatically.Notably, in the weeks following the superoutburst, K2BS5 never fully faded to its pre-superoutburst quiescent level, remaining ∼ 0.3 mag brighter after the superoutburst than it was before.As we noted in Sec.3.1, the ATLAS observations of the suspected superoutburst in 2019 also show a ∼ 0.3 mag discontinuity in the quiescent brightness level before and after the superoutburst.
The first rebrightening event was observed at BKJD=3040.3 for ∼ 2 d and showed a prominent superhump signal, while the second rebrightening occurred 20 d later.Unfortunately, the K2 campaign terminated while the second rebrightening was underway.Unlike the initial rebrightening, this second event is not accompanied by superhumps.Although the nature of the second event is somewhat ambiguous, both events appear to be causally related to the superoutburst.We base this inference on (1) the conspicuous absence of other outbursts of comparable amplitude in the longterm light curve (Fig. 1) and (2) the fact that K2BS5 was still slightly brighter than its pre-quiescent brightness.
To better understand the changes in the light curve, we used astropy (Astropy Collaboration et al. 2013) to create a two-dimensional Lomb-Scargle power spectrum (Lomb 1976;Scargle 1982) with a sliding 0.5 d window.The lower panel of Fig. 2 presents the 2D power spectrum for the full dataset, while Fig. 4 shows an enlarged version of the 2D power spectrum during the superoutburst.The steep rise of the superoutburst at BKJD = 3024 coincides with the appearance of steady oscillations with a period of ∼ 2 h, nearly identical to the orbital period.This behavior is consistent with the expected behavior of ESH in WZ Sge systems-but, for reasons we set forth in Sec. 4, we do not classify them as such.
After ∼1 day of these oscillations, the dominant frequency in the power spectrum quickly drops to a much lower frequency of 10.9 cycles/day, which we identify as the onset of Stage A superhumps.This signal increases in frequency over the two days before leveling off around 11.2 cycles/day.The superhump power fades significantly near BKJD 3034 and reemerges 1.5 d before the rebrightening.The oscillations that redevelop just before the rebrightening are seen most strongly in the second harmonic of the superhump frequency.
Fig. 5 plots the detrended light curve near the peak of the superoutburst, and it reveals that there are ∼15 cy-cles of the double-wave oscillations before the appearance of Stage A superhumps.This stage transition is rapid, occurring in just several superhump cycles.

Extinction, Absolute Magnitude, and
Superoutburst Amplitude To estimate the absolute magnitude of K2BS5, we use DECam observations taken on January 30, 2020 (Honscheid & DePoy 2008).K2BS5 was imaged in 3 filters: g, r, and i.In IRAF, aperture photometry was performed on K2BS5 and multiple surrounding stars.We find the apparent magnitude of K2BS5 in quiescence to be g = 18.01 ± 0.04, r = 17.54 ± 0.03, and i = 17.19 ± 0.03.Using the Gaia EDR3 parallax of K2BS5 in Bailer-Jones et al. ( 2021) and an reddening estimate of E(g − r) = 0.29 ± 0.02 based on a 3D dust map modeled by Green et al. (2019), we find the absolute magnitude of K2BS5 to be M g = 9.36 ± 0.15, M r = 8.88 ± 0.15, and M i = 8.54 ± 0.15, calibrated in the Gaia-SDSS-PS1 Proper Motion Catalog (Tian et al. 2017).After conversion to Gaia magnitudes 1 , we find a luminosity of G = 8.92 ± 0.13, slightly brighter than the average CV with an orbital period of 2 hours (Abrahams et al. 2020).
As seen in Figure 2, the superoutburst begins with a steep rise in flux, corresponding to a magnitude increase of ∼3.3 mag as estimated directly from the K2 data.However, given the large aperture needed to mitigate the K2 drift and the blending of K2BS5 with nearby stars, it is likely that the K2 photometry is overestimating the quiescent brightness of K2BS5.To refine our estimate of the outburst amplitude, we first converted the flux measurement at the peak of the superoutburst to an r-band magnitude.Because the effects of contamination are minimal when K2BS5 is brightest, we can assume that this inferred maximum r-magnitude is accurate.However, during quiescence, the DECam images offer a more accurate measurement of K2BS5's brightness.Using these two measurements, we find the true amplitude of the outburst to be 3.8±0.05mag.

LBT Spectrum & Ground-Based Photometry
Obtained approximately 150 days after the peak of the 2019 superoutburst, the LBT spectrum of K2BS5 (Figure 6) shows broad, double-peaked hydrogen and helium emission features typical of a quiescent dwarf nova.The continuum is relatively flat except for a significant rise at the Balmer jump.After correcting for dust extinction as described in Sec.3.3, the continuum rises slightly toward the blue, consistent with a disk dominated CV in quiescence.The full-width at half maximum (FWHM) of the H β emission line is 2020±20 km s −1 .A weak He II emission feature is seen that was not present in the Thorstensen (2020) spectrum.
The presence of He II λ4686 Å in a CV spectrum can be attributed to high-temperature plasma or photoionization, conditions generally not present in quiescent DN systems.Strong He II emission can be a sign of accretion onto a magnetic white dwarf, although here, the He II line is not especially strong.The presence of a fast-spinning WD can be tested with high cadence optical photometry, which could detect the rotational period of the WD.Because Kepler 's 30 min cadence is too slow to search for plausible spin periods, we analyzed the power spectrum of our comparatively fast-cadence SLKT observations for evidence of a short-period periodicity.We found no evidence of any such signal up to 1 https://gea.esac.esa.int a frequency of 1300 cycles d −1 .We conclude that there is no persuasive evidence that the WD is magnetized.

Mass Ratio
Theory and observation show that the stellar mass ratio in SU UMa and WZ Sge binaries is related to the ratio between the binary orbital period and the superhump period.
Following the approach of Kato & Osaki (2013a), we derive the superhump period from Stage A, and then infer the value of ϵ* from the following equation: in which ω pr represents the apsidal precession rate of the accretion disk, and ω orb represents the orbital frequency (Kato & Osaki 2013a).The orbital frequency is not discernible in the power spectrum, but it is already known from the spectroscopic study by Thorstensen (2020).Meanwhile, we measure a Stage A frequency of 10.936±0.045cycles d −1 from the power spectrum.2Combining the Thorstensen (2020) orbital period with our measurement of the Stage A period and following Kato & Osaki (2013a),3 we determine the mass ratio of the system to be q = 0.173 ± 0.035.This ratio is typical of an SU UMa-type system and would be significantly higher than most known WZ Sge-type systems (Figure 7).In WZ Sge stars, Osaki & Meyer (2002) estimate the upper limit to be q ≤ 0.08, and according to Kato (2015) most fall at or below q = 0.06.As the calculated mass ratio (q) falls well below the typical evolutionary track, we also calculated the value of q independently with the Stage B frequency to verify our Stage A measurements were not contaminated by Stage B. From the Stage B frequency of 11.160±0.089cycles d −1 , and with the method provided by Kato (2022b), we calculated the mass ratio of the system to be q = 0.175 ± 0.025.Thus, the independent calculations of q are in excellent agreement.The system's modest divergence from the typical evolutionary track in Figure 7 suggests the presence of a heavy white dwarf, which might also account for the presence of He II in Figure 6.

Observed Corrected
Figure 6.LBT quiescent spectrum of K2BS5 with no reddening correction (black line) and after correction for a reddening of E(g − r) = 0.29 mag (blue line).units are given in angstroms and flux density units in erg s −1 cm −2 Å−1 .

The First Rebrightening
The power spectrum of leading up to the first rebrightening shows significant power at the superhump frequency and its second harmonic (Fig. 2), which suggests that the disk remained tidally deformed even after the superoutburst faded.One possible explanation for the enhancement of the second superhump harmonic comes from simulations and observations by Wood et al. (2011).They showed that at the conclusion of a superoutburst, the interaction between the accretion stream and the outer disk can boost power at the second harmonic of the superhump frequency because the relative depth of the stream-disk hotspot in the WD's gravitational potential varies across the superhump cycle when the rim of the outer disk is eccentric (Wood et al. 2011).
We also see several "mini-rebrightenings," each lasting for ∼0.3 d with an amplitude of ∼0.3-0.4 mag, in the trough immediately before the first rebrightening.These mini-rebrightenings seen in Fig. 3 might be identical to the similarly named phenomenon observed in V585 Lyr by Kato & Osaki (2013c).The V585 Lyr mini-rebrightenings were also observed during a dip in the light curve immediately preceding a rebrightening, and their amplitudes, recurrence intervals, and durations were all comparable to what we see in K2BS5.The major difference is that the mini-rebrightenings in K2SB5 are comparatively ill-defined, with only two or three visible.This is far fewer than the nine very obvious mini-rebrightenings in Fig. 7 of Kato & Osaki (2013c).A similar phenomenon was also observed by Kato (2022a) in V844 Her.

An SU UMa system with some properties of WZ Sge stars
The pre-Stage-A oscillations near the superoutburst maximum are the most intriguing feature in the light curve, as they resemble ESH, the presence of which is often considered a defining quality of WZ Sge systems.As summarized in Kato (2015), ESH are low-amplitude, double-peaked modulations that occur within ∼ 0.1% of the binary orbital period; they appear near the superoutburst maximum and always precede ordinary superhumps.On one hand, the pre-Stage-A oscillations in K2BS5 appear when ESH would be expected, are consistent with the known orbital period, and have a photometric profile compatible with the compilation in Figure 11 of Kato (2015); moreover, their peak-to-peak amplitude of ∼ 0.04 mag is in excellent agreement with the histogram of ESH amplitudes in Figure 15 of Kato (2015).However, the period of the oscillations is too uncertain to establish that it matches the Thorstensen (2020) orbital period to within ∼ 0.1%, as is required of ESH (Kato 2015).As a result, we do not claim these oscillations to be ESH. 4lthough it might be tempting to dismiss the pre-Stage-A oscillations as a transient peculiarity of just one dwarf nova, Kato (2022a) reported the presence of an apparently similar phenomenon in TESS observations of the SU UMa-type dwarf nova V844 Her.Kato (2022a) noted that it is unclear as to whether double-waved oscillations are a general feature of SU UMa systems and cautioned that this phenomenon should not be confused with ESH; however, that study did not explain how to distinguish the two using photometry alone.Considering the recent recognition of long-period WZ Sge stars (Wakamatsu et al. 2017, and references therein), this point would benefit from elaboration, as the similarities between the two phenomena are sufficiently close that it can complicate classifications of WZ Sge systems based solely on time-series photometry.
A full consideration of the criteria of WZ Sge systems provides considerable evidence against K2BS5 being a WZ Sge system, despite several similarities.In addition to ESH at the beginning of the superoutburst, WZ Sge stars are generally characterized by several additional observational properties (Kato 2015): • one or more rebrightening events at the end of the superoutburst.
• absence of a distinct precursor outburst5 .
• extremely long supercycles, defined as the average time between superoutbursts, with a minimum measured duration of 4 years (Kato 2015).
• large outburst amplitudes that typically exceed 7 mag.
• orbital periods that are less than 0.065 d.
• a mass ratio, q, generally less than 0.1.
With K2BS5, it is evident from Fig. 2 that there is no distinct precursor to the superoutburst, which is a property of WZ Sge systems.The interval between supercycles, however, is a bit more ambiguous; there are large seasonal gaps in the ground-based survey photometry, and the K2 superoutburst occured during one of them.The interval between the only observed superoutbursts (in April 2017 and September 2019) suggests a supercycle of roughly 2.5 years (Figures 1 and 2).This interval would be rather short for a typical WZ Sge star, the supercycles of which typically range from 4 years to upwards of 30 years, with a majority of these systems having recurrence times shorter than ∼20 years.Nonetheless, Table 1 in Wakamatsu et al. (2017) lists two candidate long-period WZ Sge systems, BC UMa6 and V1251 Cyg, whose supercycles can be as short as 2 or 3 years, respectively.Another basic characteristic of WZ Sge-type systems is the rarity of normal outbursts during the periods between superoutbursts.The long-term light curve in Fig. 1 is compatible with this criterion, as K2BS5 appears to experience superoutbursts almost exclusively.
The argument for a long-period WZ Sge interpretation of K2BS5 begins to fall apart on other grounds.In particular, long-period WZ Sge systems are hypoth-esized to have unusually low mass-transfer rates at a given orbital period, which enables the outer disk to expand unusually far (Wakamatsu et al. 2017).Several different lines of evidence suggest that K2BS5's masstransfer rate is too high to be in this regime.First, as we noted earlier, its absolute magnitude is slightly brighter than CVs of comparable orbital period Abrahams et al. (2020).Furthermore, the presence of He II λ4686 Å in the LBT spectrum underscores that the mass-transfer rate is not especially low.The Wakamatsu et al. (2017) mechanism for producing long-period WZ Sge stars is therefore inapplicable to K2BS5.
Another argument against interpreting K2BS5 as a long-period WZ Sge system is that ESH are detectable only above binary inclinations of i ≳ 40 • (Kato 2015(Kato , 2022c)).Thus, if the pre-Stage-A oscillations were ESH, we would expect to detect the binary orbital period in the quiescent light curve.The absence of the orbital frequency in the quiescent power spectrum is consistent with a low orbital inclination.
The superoutburst amplitude (3.8±0.05mag; see Sec. 3.3) is probably the most blatant observational dissimilarity with long-period WZ Sge systems.In their Section 3.3, Kato (2015) reported that 75% of known WZ Sge systems exhibit an outburst of at least 6.9 mag, with a median value of 7.7 mag.Moreover their Figure 3, which presents a histogram of the superoutburst amplitudes of WZ Sge stars, only extends down to 5 magni-tudes, which only underscores how extraordinarily low K2BS5's amplitude is when compared to typical WZ Sge systems.
On balance, K2BS5 is best characterized as an SU UMa system that shows some deceptive observational similarities with WZ Sge systems.The nature of the pre-stage-A oscillations is unclear, but given the presence of a similar phenomenon of unknown origin in V844 Her (Kato 2022a), future Kepler -and TESS-based studies of SU UMa systems should search for this phenomenon to ascertain both its prevalence and physical origin.

CONCLUSION
K2BS5 is an SU UMa-type dwarf nova with infrequent superoutbursts, no observed normal outbursts, and a mass ratio of q = 0.173 ± 0.035.Its most notable property is the short-lived appearance of double-peaked oscil-lations near the peak of the superoutburst, prior to the emergence of ordinary superhumps.The period of these oscillations agrees (within the errors) with the spectroscopic orbital period from Thorstensen (2020) and is significantly shorter than the periods of the subsequent ordinary superhumps.Observationally, this phenomenon could easily mimic the early superhumps observed in WZ Sge systems, but their period cannot be measured with sufficient precision to test whether they agree with the orbital period to within 0.1% (a prerequisite for classifying them as early superhumps).

Figure 1 .
Figure1.The ASAS-SN, Gaia, CRTS, and ATLAS extended lightcurves from February 2005 to April 2022.The more sparse sampling shows evidence of few notable events apart from one superoutburst occurring in September 2019.There are no normal outbursts beyond superoutbursts, a hallmark of WZ Sge systems.The superoutburst observed by Kepler whose observation time is highlighted in red, is not visible because the system was near solar conjunction at the time.
under the designation

Figure 2 .
Figure2.The full Kepler /K2 light curve shown in magnitudes relative to the quiescent magnitude (top panel) and timeresolved power spectrum of K2BS5 (bottom panel).The light curve shows a long-period of quiescence before the steep rise at the beginning of the superoutburst.A decrease in magnitude near the end of the superoutburst precedes an rebrightening event before the system resumes its original state of quiescence.The time-resolved power spectrum shows that periodic variability is present only during the superoutburst.

Figure 3 .
Figure3.Low-amplitude oscillations appear during the post-superoutburst phase just before the rebrightening.These ∼0.4 mag oscillations are reminiscent of the "minirebrightenings" recorded in the Kepler data of V585 Lyr(Kato & Osaki 2013b) and the TESS data of V844 Her(Kato  2022a).

Figure 1
Figure 1 plots survey photometry of K2BS5 obtained by the Catalina Real-Time Transient Survey (CRTS; Drake et al. 2009), the All-Sky Automated Survey for Supernovae (ASAS-SN; Shappee et al. 2014; Kochanek et al. 2017), Asteroid Terrestrial-impact Last Alert System (ATLAS; Tonry et al. 2018), and Gaia.Perhaps the most striking property of these data is the absence of outbursts; only a single undisputable outburst is present

FrequencyFigure 4 .
Figure 4.The shared-axis, detrended lightcurve (top) and time-resolved power spectrum (bottom) of K2BS5 starting just before the maximum of the superoutburst.Frequency units are cycles d −1 .Double-wave oscillations near the orbital period begin slightly before BKJD 3024 and transition rapidly into stage A superhumps, marked by the frequency drop on the power spectrum mid day 3024.Stage A is also relatively short lived, rising rapidly to higher frequency stage B. While the superhumps are not present during the rebrightening event, they do reappear just before it, between BKJD 3037 and BKJD 3039.Both the fundamental and second harmonic are visible on the power spectrum.Note: different intensity cuts were used in the second and third panels of the figure to improve signal visibility of the second harmonic.

Figure 5 .
Figure 5. Top: Transition from double-wave oscillations to Stage A superhumps.The expected times of superhump maxima are indicated with black arrows (for the double-wave oscillations) and blue arrows (for Stage A superhumps).Both the doublewave oscillations and Stage A superhumps show stable, periodic maxima.Double-wave oscillations persisted for ∼15 cycles before transitioning into Stage A superhumps in just several superhump cycles.Bottom left: Power spectra of the double-wave oscillations and Stage A superhumps.Bottom right: Phase-averaged profile of double-wave oscillations.
(Wakamatsu et al. 2017 of the estimated mass ratio (q) versus binary orbital period of known WZ Sge stars.The dashed blue line shows the standard CV evolutionary track fromKnigge et al. (2011), while the solid blue line represents their optimal binary track.The dashed red line marks the short-period edge of the orbital period gap(Knigge 2006).Confirmed WZ Sge stars fromKato (2015)andKato (2022b)are labeled in red alongside short period CV stars from Kato (2022c) in grey.K2BS5 is shown in gold, while the long-period WZ Sge systems RZ Leo and ASASSN-16eg(Wakamatsu et al. 2017) are plotted as blue diamonds.