Five-colour photometry of early-type stars in the direction of galactic X-ray sources.

The results of five-color (Walraven system) photometry of 551 O- and B-type stars located in 17 fields of a few square degrees around galactic X-ray sources are presented. From a comparison of reddening-free combinations of color indices with theoretical values, calculated for model atmospheres of Kurucz (1979), effective temperature and surface gravity for these stars are derived. In addition their absolute magnitude are determined by combining these parameters with the results of evolutionary calculations of massive stars. These effective temperatures are in good agreement with the temperature scale of Bohm-Vitense (1981) for stars of luminosity classes II to V. For the supergiants the effective temperatures are about 40 percent higher. For stars of luminosity classes III to V the absolute magnitudes agree well with the results of independent luminosity calibrations of spectral types, but for brighter stars they deviate systematically. The photometric data are also used to study the interstellar reddening in the direction of the X-ray sources.