HELIUM AND CARBON-OXYGEN WHITE-DWARFS IN CLOSE BINARIES

We present a scenario model of the population of close binaries in whi ch the brighter component is a helium or a carbon-oxygen white dwarf. The other, dimmer component can be a helium white dwarf, a carbon-oxyg en white dwarf, or a low-mass (less than or equal to 0.3 M.) main-sequ ence star. The model takes into account the effects of observational s election related to the cooling of white dwarfs and the relative brigh tness of components. The total birthrate of such binaries in our Galax y is estimated to be similar to 0.17 yr(-1). In the model, in 63% of a ll cases, the dimmer component is also a white dwarf. In 82% of the sy stems that consist of two close white dwarfs, the brighter component i s a helium white dwarf. This explains why, in at least seven out of th e eight white dwarf pairs found in recent years, the primary is compos ed of helium. We estimate that close white dwarf pairs may constitute 1/9 of the total sample of observed white dwarfs. Systems that have a total mass exceeding 1.4 M. and in which the components are close enou gh to merge in a Hubble time may constitute similar to 1/40 of all clo se white dwarf pairs. This means that the sample of observed white dwa rf pairs must be at least quadrupled before one may hope to find a hyp othetical Type Ia supernova (SN Ia) precursor. The total number of SN Ia precursors is estimated to exceed the observable number by a factor of about 20. The merger frequency of close binary helium white dwarfs in the Galaxy is estimated to be similar to 0.02 yr(-1). This number is consistent with the fact that, in the total sample of known white d warf pairs, there are three in which the components are close enough t o merge in a Hubble time. In one system (WD 1101 + 364), the merger ma y result in the formation of a helium subdwarf (nondegenerate helium s tar), and in two others (WD 2331 + 290 and WD 0957 - 666), probably in the formation of a hydrogen-deficient subgiant and possibly later of an R CrB star. The predicted merger frequency is also consistent with the fact that two out of seven white dwarfs selected for their low mas s are apparently single, while five are in close binaries.