DISCOVERY OF A NEW WHITE-DWARF IN A BINARY-SYSTEM (EUVE-0720-317) IN THE EXTREME-ULTRAVIOLET EXPLORER SURVEY AND IMPLICATIONS FOR THE LATE STAGES OF STELLAR EVOLUTION

A new precataclysmic binary is identified in the Extreme Ultraviolet Explorer (EUVE) all-sky survey. The bright source EUVE 0720-317 shows a hot hydrogen-rich white dwarf optical continuum with overlying narrow Balmer-line emission. Using high signal-to-noise ratio spectroscopy in the 4100-6700 angstrom range, we identify a late-type companion and find a 1d.3 periodic modulation in the emission-line velocities and strengths. This is the signature of Feige 24-type close binary systems. We determine the components' spectral types (DAO and dM0-2), orbital velocities (K(DAO) = 104 +/- 12 km s-1, K(dM) = 96 +/- 7 km s-1), and systemic velocity (gamma = 15 +/- 12 km s-1). A first estimate of the white dwarf gravitational redshift, gamma(g) = 45 +/- 20 km s-1, and theoretical mass-radius relationships imply R(DAO) = 0.010-0.016 R. and M(DAO) = 0.55-0.90 M.. The orbital inclination is therefore i greater-than-or-equal-to 52-degrees, consistent with the large amplitude variations found in Halpha equivalent widths that imply i greater-than-or-equal-to 42-degrees. We show that the discovery of new close WD + MS binary systems in EUV sky surveys has important implications for theory of common-envelope evolution, in particular for the predicted close binary birthrate and orbital and stellar parameters.