The Puzzling Properties of the Helium White Dwarf Orbiting the Millisecond Pulsar PSR J1911–5958A in NGC 6752^*

We have used phase-resolved high-resolution images and low-resolution spectra taken at the ESO Very Large Telescope to study the properties of the low-mass helium white dwarf companion to the millisecond pulsar PSR J1911-5958A (COM J1911-5958A), in the halo of the Galactic globular cluster NGC 6752. The radial velocity curve confirms that COM J1911-5958A is orbiting the pulsar and allows us to derive a systemic velocity of the binary system nicely in agreement with that of NGC 6752. This strongly indicates that the system is a member of the cluster, despite its very offset position (~74 core radii) with respect to the core. Constraints on the orbital inclination (≳70°) and pulsar mass (1.2-1.5 M_☉) are derived from the mass ratio M_PSR/M_COM = 7.49 ± 0.64 and photometric properties of COM J1911-5958A. The light curve in the B band shows two phases of unequal brightening (Δmag ~ 0.3 and 0.2, respectively) located close to quadratures and superimposed on an almost steady baseline emission: this feature is quite surprising and needs to be further investigated.