SPECTROSCOPY OF THE RECURRENT NOVA U-SCORPII

U Sco, the recurrent nova with the shortest known recurrence time, has been argued to be a strong case for a very massive accreting white dwarf undergoing thermonuclear explosions. Such massive white dwarfs have also been proposed as the progenitors of millisecond pulsars in the disk and in globular clusters. We present optical spectroscopic observations of the system and measure both emission- and absorption-line velocities. We derive an upper limit to the mass for the white dwarf of less than or similar 0.9 M . (3-sigma) in direct contradiction to the thermonuclear instability models. Hydrogen is seen in absorption for the first time, strongly suggesting that the donor is reasonably normal. This, and the observed high helium abundance in the emission spectrum, pose additional difficulties to the accretion instability models. Thus it appears there is no viable model for this interesting source.