by radiation pressure on spectral lines (as O star winds), was in doubt for many decades, as observations of WR stars showed particularly high wind momenta with values of the product of the mass-loss rate and the terminal flow velocity, ${\mbox{$\dot{M}$}}{\mbox{$v_{\infty}$}}$, 10 – 100 times larger than that of the momentum of the stellar radiation field, $L_{*}/c$ – the so-called “wind-momentum problem” (e.g. Barlow et al. 1981), which in effect was an opacity problem (Gayley et al. 1995). Developments over the last years have shown that radiation pressure may be a viable mechanism for both initiating as well as driving the winds of WR stars after all.

One reason is the discovery that WR stars are highly clumped (e.g. Moffat & Robert 1994, Lepine & Moffat 1999), which has resulted in a downward revision of the observed WR mass-loss rates (e.g. Hamann & Koesterke 1998). A second reason is the realization that the ionization structure of WR winds is stratified: at progressively deeper layers, higher ionization stages become dominant, which increases the number of lines available for wind driving, whilst simultaneously line photons are allowed to be scattered more than once – increasing their effectiveness in transferring momentum to the 