  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. 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 degrees and that the
unseen companion is a white dwarf with mass >= 0.46 Msun. 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.
