1998A&A...334...21V

We present the results of population number synthesis calculations on young starbursts in regions of solar metallicity (Z=0.02), using a Monte Carlo simulation of bursts containing 1000 stellar objects. They clearly show that one has to account for interacting binaries to get a correct image of these regions. Independently of all sorts of distributions such as initial mass function (IMF) or mass ratio distribution, interacting binary evolution makes starbursts look younger than they really are. What happens is that after some time, the area of the main sequence (MS) above the actual turnoff point of the Hertzsprung-Russell Diagram (HRD) gets crowded with accretion stars, which are part of the end product of mass transfer in close binaries. Shortly after their appearance, these accretion stars (blue Stragglers) begin to dominate the O star population in a starburst, proceeding to the moment where they are the only ones left (since at that particular stage, all the O type single stars and binary O type primaries have disappeared). As a consequence, if it is assumed that the entire population are single stars, or that the features of the starburst are solely due to single star evolution, the resulting age estimation of the starburst is wrong, i.e. the starburst may be considerably older than it appears.