of the mass. If the wind density becomes lower, the coupling between active and passive ions may disappear, potentially giving rise to multicomponent effects (e.g. Lucy & Solomon 1970; Castor et al. 1976). In O stars of Galactic metallicity, this ion-decoupling will only occur in the outermost regions of the wind, where the flow reaches its asymptotic velocity – and the mass-loss rate is not affected.

For the lower density winds at extremely low $Z$, ion-decoupling may become important for the mass-loss rate. It has been proposed that runaway instabilities may occur in a multi-component wind (Owocki & Puls 2002, Krtička & Kubát 2002), and possibly even fallback of the passive ions. Krtička et al. (2003) present a relation defining the mass loss – metallicity domain where multicomponent effects become relevant. Note that their diagram is intended for normal O stars. Ignoring this fact, and applying this diagram for the WR model parameters discussed in our study indicates that multicomponent effects start to play a role below a metal content of $Z/{\mbox{$Z_{\odot}$}}$ $\sim 1/300$. However, two remarks are in order: first, at low $Z$ the relative contribution of hydrogen and helium to the line force will increase (see Vink et 