Abstract
The bright, K1 III-IV star γ Cep has been reported previously to have a possibly substellar companion in a ~2.5 yr orbit, as well as an unseen stellar companion at a larger separation. We determine for the first time the three-dimensional orbit of the latter, accounting also for the perturbation from the closer object. We combine new and existing radial velocity measurements with intermediate astrometric data from the Hipparcos mission (abscissa residuals), as well as ground-based positional observations going back more than a century. The orbit of the secondary star is eccentric (e = 0.4085 ± 0.0065) and has a period P = 66.8 ± 1.4 yr. We establish the primary star to be on the first ascent of the giant branch and to have a mass of 1.18 ± 0.11 M☉, an effective temperature of 4800 ± 100 K, and an age around 6.6 Gyr (for an assumed metallicity [Fe/H] = +0.01 ± 0.05). The unseen secondary star is found to be an M4 dwarf with a mass of 0.362 ± 0.022 M☉ and is expected to be ~6.4 mag fainter than the primary in K. The minimum mass of the putative planetary companion is Mp sin i = 1.43 ± 0.13 MJup. Based on high-precision Hipparcos observations, we are able to place a dynamical upper limit on this mass of 13.3MJup at the 95% confidence level, and 16.9MJup at the 99.73% (3 σ) confidence level, thus confirming that it is indeed substellar in nature. The orbit of this object is only 9.8 times smaller than the orbit of the secondary star (the smallest ratio among exoplanet host stars in multiple systems), but it is stable if coplanar with the binary.
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