THE ORBITAL PERIOD OF THE PRE-CATACLYSMIC BINARY RE-2013+400 AND A STUDY OF THE ATMOSPHERE OF THE DAO WHITE-DWARF PRIMARY

Several pre-cataclysmic binaries, comprising a hot white dwarf with a red dwarf companion, have been discovered as a result of the optical identification of EUV sources from the ROSAT all-sky survey. The optical spectra have the steep blue continuum and Balmer absorption typical of a hot white dwarf, but there are bright, narrow emission lines of H I (and sometimes He I and Ca II) superimposed. An intense campaign of follow-up observations has been devoted to these binary systems. So far, only RE 2013 + 400 has exhibited any measurable changes in the radial velocities of the emission components, from which it is possible to estimate that the binary period is 0.71 d. A clear Hell 4686-Angstrom absorption feature is detected, which indicates that, like most PCBs with white dwarfs hotter than 40 000 K, the white dwarf is a hydrogen-helium hybrid star (DAO). A combined analysis of the optical, UV and EUV/X-ray data suggests that the atmospheric He abundance is higher in the optical line-forming region of the white dwarf photosphere than in the region where the EUV/X-ray flux is formed. This is an interesting result, in the light of the recent optical study of a sample of DAO white dwarfs by Bergeron et al., if representative of DAO white dwarfs in general.