Optical observations of compact galactic X-ray sources

The author presents a comprehensive review of the optical observations of bright galactic X-ray sources (L_x ≥ 10³⁵ erg s^-1) containing neutron stars (or perhaps black holes). Two classes of objects are distinguished: (1) massive X-ray binaries (MXRB), where the companion star is a massive (M ≥ 10 M_sun;) early-type star which transfers mass to the compact object by a stellar wind or by incipient Roche-lobe overflow; (2) low-mass X-ray binaries (LMXB), which are composed of a neutron star and a low-mass (M ≤ 1 M_sun;) star, which transfers matter by Roche-lobe overflow. This group comprises the bright galactic bulge sources, X-ray bursters, globular cluster sources and soft X-ray transients.