Limits on the space density of double degenerates as type IA supernova progenitors

A search for variable radial velocities in 25 hot white dwarfs yielded no binaries with orbital periods between 3 and 10 hr. The fraction of all white dwarfs with spectral classes DA1 to DA5 that are binary with periods in this range is less than or equal to 0.12, with 90 percent confidence. A simple model to predict the merger rate of binary degenerates, based on the observations presented on the observed luminosity function of white dwarfs, on assumptions about the distribution of initial binary periods, and on estimates of the fraction of all white dwarf pairs whose combined mass exceeds the Chandrasekhar limit. The upper limits so derived are less than the observed rate of Galactic Type Ia supernovae.