Final mass (at core collapse) as a function of initial mass for helium star models of different metallicities. The dots on each line indicate, for each metallicity, the initial helium star mass at which the transition between WN- and WC-type mass loss is observed to occur in the models. The rhombi indicate the value of the minimum luminosity WN-type models obtained from Eq. (8), and using the mass-luminosity relation of Langer (1989). Helium stars below this limit do not have optically thick WR winds and have overestimated mass-loss rates.