Abstract
Theoretically, the evolution of a young stellar population depends on metallicity at least through two very important effects: the increasing opacity of the stellar material and the dependence of mass loss on metal content in high mass stars. As a consequence of the first, the effective temperature of ionizing stars should be lower in regions of higher metallicity (see for example McGaugh 1991) but, on the other hand, as a consequence of the second, that is if the strength of stellar winds increases with metallicity, the loss of the outer envelopes of the most massive stars can increase their surface temperature to very high values. These highly evolved massive O stars are identified with the Wolf-Rayet population.
