AN IMPROVED ORBITAL PERIOD FOR THE RECURRENT NOVA U-SCORPII

U Sco is a recurrent nova as well as a double-lined eclipsing binary. As such, an accurate orbital period before its next eruption can be co mbined with a measured period after that eruption to yield a dynamical measurement of the ejected mass. A properly phased radial velocity cu rve for both components could yield the mass of the white dwarf, which theory strongly predicts to be near the Chandrasekhar mass. Radial ve locity measurements from three epochs were available but until now cou ld not be phased together reliably. Both programs require a highly acc urate orbital period. We report on eclipse photometry from 1988 to 199 4 with times for nine eclipses. The eclipse light curve is variable in shape and asymmetric with the egress slower than the ingress, implyin g a bright and variable hot spot. We derive an orbital period of 1.230 5631+/-.0000030 days and an eclipse minimum time of HJD 2,447,717.6061 +/-0.0032. Phasing the radial velocity data shows a large scatter in b oth the emission-and absorption-line velocities, which have significan tly different averages. This casts doubt on previous mass determinatio ns for the white dwarf, in particular the claim that the white dwarf i s much less than the Chandrasekhar mass.