1998NewA....3..443V

Evolutionary calculations of massive single stars and of massive close binaries that we use in the population number synthesis (PNS) code are presented. Special attention is given to the assumptions/uncertainties influencing these stellar evolutionary computations (and thus the PNS results). A description is given of the PNS model together with the initial statistical distributions of stellar parameters needed to perform number synthesis.We focus on the population of O-type stars and WR stars in regions where star formation was continuous in time and in starburst regions. We discuss the observations that have to be explained by the model. These observations are then compared to the PNS predictions.We conclude that: . probably the majority of the massive stars are formed as binary components with orbital period between 1 day and 10 yr; most of them interact. . at most 8% of the O-type stars are runaways due to a previous supernova explosion in a binary; recent studies of pulsar space velocities and linking the latter to the effect of asymmetrical supernova explosions, reveal that only a small percentage of these runaways will have a neutron star companion. . with present day stellar evolutionary computations, it is difficult to explain the observed WR/O number ratio in the solar neighbourhood and in the inner Milky Way by assuming a constant star formation rate, with or without binaries. The observed ratio for the Magellanic Clouds is better reproduced. . the majority of the single WR stars may have had a binary past. is better reproduced. . the majority of the single WR stars may have had a binary past. . probably merely 2-3% (and certainly less than 8%) of all WR stars have a neutron star companion. . a comparison between theoretical prediction and observations of young starbursts is meaningful only if binaries and the effect of binary evolution are correctly included. The most stringent feature is the rejuvenation caused by mass transfer.