RE 0720-318 - A PRE-CATACLYSMIC BINARY-SYSTEM WITH A DAO WHITE-DWARF PRIMARY

A number of pre-cataclysmic binaries, comprising a hot white dwarf wit h a red dwarf companion, have been discovered as a result of the optic al identification of EUV sources from the ROSAT all-sky survey. From t he detection of narrow emission lines in recently recorded optical spe ctra, we report here the discovery that RE 0720 - 318 (listed just as a DA white dwarf in the Wide Field Camera Bright Source Catalogue) is a further example of such a system. A He II 4686-Angstrom absorption f eature indicates that the white dwarf is a hydrogen-helium hybrid DAO star. This is only the third DAO to be detected as an EUV/X-ray source , and it is remarkable that ah three reside in pre-cataclysmic binarie s. From follow-up optical photometry coupled with emission-line radial velocity and intensity measurements from the spectral data, we obtain an orbital period of 1.201 d.