Population of stars with a surface hydrogen mass fraction of X_surf < 0.4, i.e. helium stars, as a function of time assuming an initial population of 10⁶ M_⊙, compared to the photon flux Q(O VI). Those with very low hydrogen (X_surf < 0.001, classic Wolf-Rayet stars) are subdivided by the presence (WC) or absence (WN) of carbon. The population is further subdivided into high luminosity (solid lines) and low luminosity (dot-dash) populations. We show results at the highest and lowest metallicities considered here.