Simultaneous light and velocity curve solution and probable starspot activities for the W UMa system U Pegasi

A statistical study is presented on the distribution of different kinds of massive star classes in the HR diagram using recent calculations of massive single star and massive close binary evolution. The influence of the mass transfer process during the critical Roche lobe overflow phase of a primary component on the relative frequency of the different classes is outlined. It is shown that without an exact knowledge of how mass transfer takes place, the meaning of an initial mass function determined by classical methods is unclear whereas a direct comparison of the observed and theoretically predicted blue/red star ratio is meaningless. The number of O- and WR-type stars with compact companions is expected to be very low (<5%). If mass transfer in binaries is largely conservative, the contribution of real single stars to the supernova II population is low (10–25%).