Evolutionary stellar population synthesis models have been performed for several metallicities and two extreme star formation rates, instantaneous (IB) and extended bursts (EB). We discuss the dependence on metallicity of the population of Wolf-Rayet (WR) and Red Supergiant (RSG) stars. We show that both populations become more abundant for higher metallicities. We also show the effects of metallicity on the effective temperature and Hβ equivalent width. These effects are independent of the IMF slope and can account, at least in part, for the higher values of Teff and W(Hβ) systematically found in low-metallicity star formation episodes.

A more complete study can be found in Cerviño & Mas-Hesse (1994).

Wolf-Rayet population

In Figure 1a we compare observational values of WR bump over Hβ ratio taken from Kunth & Joubert (1985), Kunth & Schild (1986) and Vacca & Conti (1992) with the model predictions. Ages have been estimated by using the W(Hβ) computed for a Salpeter IMF slope (Figure lb). The average WR bump over Hβ intensities fall within the range predicted by our models for an IB regime and cannot be reproduced assuming an extended one. We can therefore reject the possibility of having extended star formation episodes in the majority of the cases, and can also constrain the age of the episodes to a short range between 3 and 5 Myr after the onset of the burst.