the other metal species (S, Cl, Ar, etc.) in accordance to their solar values. In the WN case, we assume that the CNO-cycle affects the relative abundances of C, N, and O in amounts that are similar to our study for Luminous Blue Variables (LBVs; Vink & de Koter 2002 and references therein), so that the total sum of C+N+O abundance is constant, and scales with the iron abundance.

We note that the stellar and ISM abundances are equal for elements such as Fe, but the stellar and ISM carbon abundance can be vastly different. It is therefore important to be very careful with our definition of $Z$. We also note that extragalactic metallicities are usually derived from nebular oxygen lines. The ISM oxygen and Fe abundances do not necessarily scale with the solar values, due to the differing roles of SNIa, which release Fe, and SN II, which release large amounts of $\alpha$ elements, but less Fe. \[$\alpha$/Fe\] scalings can be adjusted using Table 5 by Vink et al. (2001).

The strengths of our approach are the natural way in which multiple line and continuum interactions are accounted for, and the consistent computation of an ionized stratification throughout the 