AN EPHEMERIS FOR THE PRE-CATACLYSMIC BINARY EUVE J0720-317

We present H alpha radial velocities and equivalent widths of the hot white dwarf plus red dwarf binary EUVE J0720-317, which cover a 621 da y baseline without cycle-count ambiguities and define an orbital perio d of 1.26245 +/- 0.00004 d. The equivalent width variation lags the ra dial velocity by 0.21 +/- 0.02, consistent with an expected 0.25-cycle offset if the emission arises entirely from reprocessing of the white dwarf's EUV radiation in the red dwarf upper atmosphere. Moreover, us ing our new ephemeris we show that photometric variations reported in the literature are strictly in phase with the Balmer line strength var iations, providing further support for the EUV-illumination model. We present revised estimates of the component masses and discuss the bina ry evolutionary status. (C) 1996 American Astronomical Society.