for WR stars has been studied by Crowther et al. (2002). Comparing WR stars in the Galaxy and the Large Magellanic Cloud, they report a tentative (Z) dependence, however this result is not clear-cut, in part because the investigated baseline in $Z$ is rather narrow (about a factor of two in $Z$).

As WR stars produce copious amounts of carbon and oxygen, one might argue (referring back to the second assumption mentioned earlier) that WR mass loss is likely to be [*independent*]{} of the initial chemical composition (which is set by the metallicity of the host galaxy), once intermediate mass elements become abundant in the outer layers. The relevant question is therefore whether the lines of intermediate elements do indeed dominate the Fe lines in their contribution to the total line force, and if so, whether this holds true for all metallicities, i.e. Fe mass fractions. Or alternatively, whether the Fe lines may take over the line driving starting at some minimum value of $Z$.

For O stars, the relative importance for the wind driving of intermediate mass elements (mainly CNO) versus that of heavy metals (notably Fe) as a function of $Z$ has been studied in depth by Vink 