We present spectra of the pulsating sdB star KPD 1930 + 2752 which confirm
that this star is a binary. The radial velocities measured from the Ha and
He I 6678-Angstrom spectral lines vary sinusoidally with the same period (2
h 17 min) as the ellipsoidal variability seen by Billeres et al. The ampli
tude of the orbital motion (349.3 +/- 2.7 km s(-1)) combined with the canon
ical mass for sdB stars (0.5 M.) implies a total mass for the binary of 1.4
7 +/- 0.01 M.. The unseen companion star is almost certainly a white dwarf
star. The binary will merge within similar to 200 million years because of
gravitational wave radiation. The accretion of helium and other elements he
avier than hydrogen on to the white dwarf, which then exceeds the Chandrase
khar mass (1.4 M.), is a viable model for the cause of Type Ia supernovae.
KPD 1930 + 2752 is the first star to be discovered that is a good candidate
for the progenitor of a Type Ia supernova of this type, which will merge o
n an astrophysically interesting timescale.