RE 0720–318: a pre-cataclysmic binary system with a DAO white dwarf primary

Abstract

A number of 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. From the detection of narrow emission lines in recently recorded optical spectra, 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 HeII 4686-Å absorption feature 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 all three reside in pre-cataclysmic binaries. 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.