A ROSAT SURVEY OF HOT DA WHITE-DWARFS IN NONINTERACTING BINARY-SYSTEMS

A number of new non-interacting binary systems comprising a white dwarf plus a normal stellar companion have been discovered by the ROSAT X-ray and EUV sky surveys. We discuss the identification of nine of these objects, determine the temperatures and gravities of the white dwarfs, and compare their atmospheric compositions with the population of isolated white dwarfs. As a result, it may be possible to estimate how many hot white dwarfs in total reside in binary systems. Coronal emission is detected in several systems, and its nature, in the light of the likely age of the binaries, is discussed. These binary systems are important representatives of several possible evolutionary paths, including common-envelope and mass-transfer phases. Their further study will be of great importance in understanding stellar evolution in general, and binaries in particular.