Detection of a companion to the pulsating sdB Feige 48 ⋆ , ⋆ ⋆

We present the discovery of a binary companion to the pulsating sdB Feige 48. Using HST/STIS and archival FUSE spectra, we measure a period of 0.376±0.003 d and a velocity semi-amplitude of 28.0±0.2 km s −1. This implies that the companion star must either be of very low mass, or the orbit is at high inclination. Combining 2MASS fluxes, the lack of a reflection effect, results from asteroseismology and a measurement of the rotation velocity of Feige 48, we show that the orbital inclination must be ≤11.4 • and that the unseen companion is a white dwarf with mass ≥0.46 M⊙. The implications of this discovery, and of binarity amongst sdB pulsators, is then discussed in the context of recent theoretical work on sdB formation. In particular we suggest that radial velocity studies focus on sdB pulsators with no known companion, and that asteroseismological studies of sdBs investigate a larger mass range than previously considered in order to test formation models.