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 Hα and Hei 6678-Å spectral lines vary sinusoidally with the same period (2h 17min) as the ellipsoidal variability seen by Billères et al. The amplitude of the orbital motion (349.3+/-2.7kms^-1) combined with the canonical mass for sdB stars (0.5M_solar) implies a total mass for the binary of 1.47+/-0.01M_solar. The unseen companion star is almost certainly a white dwarf star. The binary will merge within ~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 (1.4M_solar), 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 time-scale.