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

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