The Brussels PNS-Code takes a realistic fraction of binaries into account. Finally the evolution of the spectrum of a young stellar aggregate is plotted as a function of age. The WR stars show of course broad and strong emission lines superimposed upon a weaker continuum. The variation of the spectrum with time depends on basic assumptions which can be made upon the IMF, the fraction of binaries and the initial distribution of primary masses, periods and mass fractions herein. Especially the presence of binaries changes the the spectrum drastically at some epochs.
Binaries rejuvenate the spectrum of the stellar aggregate.
Figure 3 shows the variation with time of the spectrum of a starburst containing 10.000 stars in the case that no binaries are present in the stellar aggregate. The single stars follow the evolutionary tracks as calculated by Schaller at al. The evolution of each binary in the sample was calculated separately with the Brussels binary evolution code.