Evolutionary constraints on the masses of the components of HDE   226868/Cyg X-1 binary system
  Calculations carried out to model the evolution of HDE 226868, under different assumptions about the stellar wind mass loss rate, provide robust limits on the present mass of the star. It has to be in the range 40 +- 5 solar masses if the distance to the system is in the range 1.95 to 2.35 kpc and the effective temperature of HDE 226868 in the range 30000 to 31000 K. Extending the possible intervals of these parameters to 1.8 to 2.35 kpc and 28000 to 32000 K, one gets for the mass of the star the range 40 +- 10 solar masses. Including into the analysis observational properties such as the profiles of the emission lines, rotational broadening of the absorption lines and the ellipsoidal light variations, one can estimate also the mass of the compact component. It has to be in the ranges 20 +- 5 solar masses and 13.5 to 29 solar masses for the cases described above. The same analysis (using the evolutionary models and the observational properties listed above) yields lower limit to the distance to the system of about 2.0 kpc, if the effective temperature of HDE 226868 is higher than 30000 K. This limit to the distance does not depend on any photometric or astrometric considerations. 
J. Ziolkowski (N. Copernicus Astronomical Center, Warsaw, Poland)