The luminosity excess of OB stars in quasi-stationary X-ray binaries

We have found a mass—luminosity relation for the OB components of massive X-ray binaries that is a good estimator of the masses of these evolutionarily important binaries. Analysis of this relation showed a systematic luminosity excess of ≈1m for the OB components in these systems. No such excess was discovered for the evolutionarily related WR + O binaries, which also undergo mass exchange between their components and are the immediate precursors of X-ray binaries. A study of possible origins of the luminosity excess suggests that the most likely explanation is an X-ray luminosity related selection effect for massive X-ray binaries. Estimates show that the probability of detecting X-ray binary increases due to the enhancement of the stellar wind, which increases the efficiency of accretion by the relativistic companion while the optical component evolves along the main sequence. This can explain the magnitude of the observed luminosity excess and the position of the optical components of X-ray binaries in the luminosity—radius plane.