KPD 1930+2752: a candidate Type Ia supernova progenitor

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 amplitude of the orbital motion (349.3 +/- 2.7 km s(-1)) combined with the canonical mass for sdB stars (0.5 M.) implies a total mass for the binary of 1.47 +/- 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 heavier than hydrogen on to the white dwarf, which then exceeds the Chandrasekhar 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 on an astrophysically interesting timescale.