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We present new high-resolution, high signal-to-noise optical spectra of HD 77581, the optical counterpart of the X-ray source Vela X-1, and determine radial velocities from these spectra, as well as from high-resolution IUE spectra and from digitized photographic spectra. The measured velocities show strong deviations from a pure Keplerian radial-velocity curve, which are autocorrelated within one night, but not from one night to another. Since lines of different ions exhibit very similar changes in profile, these deviations most likely reflect large-scale motions of the stellar surface akin to non-radial pulsations. A possible cause could be that the varying tidal force exerted by the neutron star in its eccentric orbit excites high-order pulsation modes in the optical star which interfere constructively for short time intervals. The effect of such velocity excursions on the orbital solution is estimated by means of a Monte-Carlo simulation technique. We investigate sources of systematic error, due to, e.g., the tidal deformation of the star, and find, in particular, evidence for a systematic perturbation of the radial velocity near the time of velocity minimum. This possible distortion severely compromises the accuracy of the radial-velocity amplitude, leading to a 95\% confidence range of 18.0--28.2\,\kms. The corresponding 95\% confidence limits of the masses are given by $\MX=1.9_{-0.5}^{+0.7}\,\Msun$ and $\Mopt=23.5_{-1.5}^{+2.2}\,\Msun$., Comment: 14 pages, incl. 7 out of 8 figures, of compressed, uuencoded postscript. Accepted for publication in A&A. For 8th figure (1Mbyte), or in case of problems, contact mhvk@astro.caltech.edu (paper replaced because figure was too large)

We analyze the currently available observations of X-ray binaries in a consistent way, to re-determine the masses of the neutron stars in these systems. In particular, our attention is focussed on a realistic and consistent assessment of observational uncertainties and sources of systematic error. Confidence limits for these new mass estimates are generally less optimistic than previously assumed. The available observations, including data on six radio pulsars, do not firmly constrain the equation of state of neutron star matter. In particular, a firm upper mass limit cannot yet be established. An improvement of the accuracy of optical data holds the key to further progress., Comment: 5 pages of compressed, uuencoded postscript (text+figure). Accepted for publication in A&A. In case of problems, contact mhvk@astro.caltech.edu

Wray 977, the B supergiant companion of the X-ray pulsar GX301-2, should be classified as B1~Ia+, based on a comparison of its optical spectrum to that of zeta1 Sco, a well-known B hypergiant and ONE OF THE BRIGHTEST STARS IN THE GALAXY. The classification of Wray 977 as a hypergiant results in a new distance determination of the binary system, i.e. 5.3 kpc (previously 1.8 +/- 0.4 kpc). The ``average'' X-ray luminosity of the pulsar is then ~10^37 erg/s, in good agreement with the predicted X-ray luminosity resulting from accretion of a dense, low-velocity (v_infty = 400 km/s) stellar wind. A mass-loss rate of <= 10^-5 M_sun/yr is estimated from the Halpha profile. A new upper limit for the inclination of the system is derived which provides a lower limit to the (present) mass of Wray 977 (48 M_sun). Regarding current binary evolution scenarios, the empirical lower mass limit for BLACK-HOLE formation in a binary increases to M_zams >= 50 M_sun., Comment: uuencoded compressed postscript with figures, 7 pages

From a radial velocity study of lines of He I and the heavier ions of HD 77581 (=Vela X-1), orbital elements for this X-ray binary system are derived. Together with the orbital elements given by Rappaport and McClintock from X-ray pulsar results, this enables determination of masses for both the X-ray and the early type supergiant component.

The discovery of a faint X-ray source at approximately 282 deg galactic longitude, -9 deg galactic latitude with the rotation modulation collimator X-ray detectors aboard SAS 3 is reported. The source is designated 2S 0921-630 and is tentatively identified, on the basis of an optical search, with a 17th magnitude star exhibiting the He II line at 4686 A and H-beta in emission. The optical spectrum of this star is shown to be similar to those of the optical counterparts of Sco X-1 and other such galactic X-ray sources. It is suggested that 2S 0921-630 may belong to the class of low-luminosity galactic X-ray binaries.