  We present new atmosphere models for Wolf-Rayet stars that include a
self-consistent solution of the wind hydrodynamics. We demonstrate that the
formation of optically thick WR winds can be explained by radiative driving on
Fe line opacities, implying a strong dependence on metallicity (Z). Z-dependent
model calculations for late-type WN stars show that these objects are very
massive stars close to the Eddington limit, and that their formation is
strongly favored for high metallicity environments.
