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S. Vennes and J.R. Thorstensen
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 day baseline without cycle-count ambiguities and define an orbital period of 1.26245 +/- 0.00004 d. The equivalent width variation lags the radial 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, using our new ephemeris we show that photometric variations reported in the literature are strictly in phase with the Balmer line strength variations, providing further support for the EUV-illumination model. We present revised estimates of the component masses and discuss the binary evolutionary status.
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