title: The 0.8 day orbit of the precataclysmic binary EUVE J1016-053.
abstract: The optical counterpart of the new extreme ultraviolet source EUVE J1016-053 (=RE 1016-053) is known to show intermittent sharp Balmer and HE I emission and traces of an M dwarf associated with a white dwarf optical spectrum.  We present extensive optical spectroscopy showing the emission lines vary in velocity and intensity on a period of 0d.78929 +/- 0d.00003.  The phase of the emission-line strengths lags that of the velocities by 0.237 +/- 0.013 cycle, consistent with the quarter-cycle offset expected if emission arises from the side of the secondary facing the hot white dwarf.  EUVE J1016-053 is another example of an extreme ultraviolet illumination-effect binary, which only recently emerged from a common-envelope phase of binary evolution.  Based on spectroscopic measurements and new BVR photometry, we find that the M dwarf secondary's contribution to the combined light is smaller than previously reported, probably because of the difficulty of avoiding contamination from a third star 3".2 distant.  The velocity of the weak H II absorption is in antiphase to the emission, suggesting that it originates in the white dwarf photosphere.  The mass function implied by the emission-line motion is f(M) = 0.28 +/- 0.08 M solar.  Assuming the He II absorption does follow the white dwarf, the mass ratio MWD/MdM is 1.8 +/- 0.5, and the gravitational redshift is 45 +/- 14 km s-1.  The modulation of the emission lines suggests the inclination i > 40 degrees, but plausible masses demand this inclination be well above the value.
authors.affiliation: Department of Physics and Astronomy, Dartmouth College, Hanover, NH 03755, USA.
authors.forename: J R
authors.initials: JR
authors.lastname: Thorstensen
authors.forename: S
authors.initials: S
authors.lastname: Vennes
authors.forename: S
authors.initials: S
authors.lastname: Bowyer
mesh_terms: D055580:Astronomical Phenomena; D001256:Astronomy; D002244:Carbon / Q000032:analysis; D005118:Extraterrestrial Environment; D006371:Helium / Q000032:analysis; D008027:Light; D013057:Spectrum Analysis; D014466:Ultraviolet Rays
chemical_list: D006371:Helium; D002244:Carbon