U Sco, the recurrent nova with the shortest known recurrence time is studied by means of optical spectroscopic observations of the system, and both emission- and absorption-line velocities are measured. An upper limit to the mass for the white dwarf of approximately less than 0.9 solar mass is derived which is 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.
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Johnston et al. (1992) studied this question.