On the evolution of low mass binary systems

Eight models of the evolution of close binary systems containing a neutron star (white dwarf) and red dwarf of spectral class later than K0 are calculated, with allowance for mass loss from the system and the asynchronism of red-dwarf rotation. It is shown that if the initial mass loss ratio q sub 0 is less than approximately 2, then, in the course of approximately 10-million years, a mass-loss instability arises and a bright X-ray source forms with luminosity close to the Eddington luminosity. If q sub 0 is greater than 2, the bright X-ray source forms twice during the mass-loss phase. The calculated models explain the existence of bright X-ray sources in the galactic bulge, X-ray bursters, and their observed peculiarities (high X-ray luminosities and the lack of eclipses). They also explain the existence of the 'forbidden' interval of orbital periods and the absence of systems with periods less than approximately 80 min.