The low mass X-ray binary 2A 1822-37.1.

The optical counterpart of the X-ray source 2A 1822-371 has been followed spectroscopically through its 5.6 hr period. Velocity variations of a weak, rather broad He II 4686 A emission show that it must be formed primarily in a disk surrounding the degenerate star. The line profile variation additionally suggests the presence of an emission hot spot or stream, reminiscent of those observed in cataclysmic variables. Evidence is presented that the observed weak absorption features may be formed on the heated face of the nondegenerate companion. The primary appears to be a neutron star, probably of about 1 solar mass. The secondary has a mass of about 0.3 M solar mass, but its radius appears to be about twice as large as that of a main-sequence star of that mass.