Binaries with initial component masses in the range 7-10 stellar mass can evolve into systems consisting of a massive degenerate dwarf which is accreting pure helium at the rate of approximately 10-6 to 10-5 stellar mass/yr from a massive companion, whose precursor first fills its Roche lobe as a giant with a deep convective envelope. The mass-exchange rate in these 'helium Algols' is precisely in the interval which allows the degenerate dwarf to burn the accreted helium in a quasi-stationary way. Thus, some massive dwarfs have a realistic possibility of reaching the Chandrasekhar limit and exploding as a supernova. In our Galaxy, the frequency of such supernovae may be approximately 1.7 x 10-3/yr. During the phase of quasi-stationary helium shell burning, helium-accreting dwarfs are very bright (L approximately = 1037 - 1038 erg/s, stationary or flaring ultrasoft X-ray sources like those discovered recently in galaxies of the Local Group. Cataclysmic variable-like (CV-like) systems in which hydrogen-rich matter is transferred on the thermal timescale of the envolope of an envolved main-sequence donor can also contribute to the population of ultra-soft X-ray sources. We discuss the properties of such systems and conclude that futher observational studies are needed to determine the relative contribution of helium Algols and of CV-like systems to the family of ultrasoft X-ray binaries.
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Icko et al. (1994) studied this question.