Determination of the masses of the components of the HDE 226868/Cyg X-1 binary system

A recent determination of the distance to the HDE 226868/Cyg X-1 binary system and a more precise determination of the effective temperature of HDE 226868 allow for a more accurate estimate of the masses of both components. Using up-to-date evolutionary models, I obtain a mass range of between 25 and 35M(circle dot) for the mass of the supergiant and between 13 and 23M(circle dot) for the mass of the black hole. By accepting more liberal estimates of uncertainties in both the distance and the effective temperature, it is possible to extend these ranges to 21-35 and 10-23M(circle dot) for both masses, respectively. The most likely values within these ranges are 27 and 16M(circle dot), respectively. The mass obtained for the black hole agrees with the value of 15 +/- 1M(circle dot) suggested by Orosz et al. However, their value of 19 +/- 2M(circle dot) suggested for the mass of the supergiant should not be used because such a star violates the mass-luminosity relation for the massive core hydrogen-burning stars. This consideration was not incorporated into the iterative process of Orosz et al. To resolve this violation, I consider the possibility that the hydrogen content of HDE 226868 might be lowered as a result of the mass transfer and the induced fast rotation of the mass gainer. I have analysed the evolutionary effects of such a situation and have found that, while important, these do not invalidate the conclusions listed above. If, as a result of the rotation-induced mixing, the present hydrogen content of HDE 226868 is equal to about 0.6 (as suggested by some observational data), then its present mass might be lower: similar to 24M(circle dot) rather than similar to 27M(circle dot).