A detached double degenerate with a 1.4-h orbital period

We have discovered that the detached double degenerate binary WD 0957 — 666 has an orbital period of 1.46 h, rather than the 1.15-d orbital period reported earlier. This is the shortest period example of such a system yet discovered. We obtain a unique period, which fits both our data and earlier data. At this period the emission of gravitational radiation will cause the binary to merge within approximately 2.0 × 108 yr. This system represents a population of short orbital period binaries which will merge within a Hubble time, and so could account for type Ia supernovae, although, due to the low mass of both stars (0.3 to 0.4 M⊙), it is unlikely to become a supernova itself. We have detected the companion star and have measured a mass ratio of q = 1.15 ± 0.10. This is the third double degenerate for which q has been measured, and all three have q ≃ 1, which is in conflict with the predicted mass ratio distribution, which peaks at 0.7. This system is viewed close to edge-on, and we estimate that the probability of this system undergoing eclipses is 15 per cent.