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 6678Å spectral lines vary sinusoidally with the same period (2 h 17 m) as the ellipsoidal variability seen byBilì eres et al. (2000). The amplitude of the orbital motion (349.3±2.7 km s −1) combined with the canonical mass for sdB stars (0.5M ⊙) implies a total mass for the binary of 1.47±0.01M ⊙. The unseen companion star is almost certainly a white dwarf star. The binary will merge within ∼ 200 million years due to gravitational wave radiation. The accretion of helium and other elements heavier than hydrogen onto the white dwarf which then exceeds the Chandrasekhar mass (1.4M ⊙) is a viable model for the cause of Type Ia supernovae. KPD 1930+2752 is the first star to be discovered which is a good candidate for the progenitor of a Type Ia supernova of this type which will merge on an astrophysically interesting timescale.