THE ORBIT OF THE SUPERGIANT COMPONENT OF VELA X-1 DERIVED FROM IUE RADIAL-VELOCITIES

47 short-wavelength, high-resolution IUE spectra have been used to der ive the orbital elements of HD 77581, the supergiant companion to the neutron star in the X-ray source Vela X-1; the velocity amplitude is a t the lower end of the range found by previous workers using primarily optical spectra, but other elements (e, w) are in fair agreement with those derived from pulse timing studies of the X-ray source. The new results indicate that the mass of the neutron star is close to 1.4 M. and is not significantly larger than those measured for other members of this class of object.