The influence of binaries on the number statistics and population synthesis of large groups of stars.

Spectra of galaxies are nowadays made with high S/N ratio. The dominant characteristics of these spectra are produced by the most massive stars in that galaxy. So that we can aim to determine the fraction of each type of massive star, from the observed spectrum of the galaxy. The gala contains a substantial fraction of binaries apart from the naturally born single stars. Different types of massive stars are the result of the com- bined but mutually very different evolution of single stars, resp. binaries.