  Classical Wolf-Rayet (WR) stars mark an important stage in the late evolution
of massive stars. As hydrogen-poor massive stars, these objects have lost their
outer layers, while still losing further mass through strong winds indicated by
their prominent emission line spectra. Wolf-Rayet stars have been detected in a
variety of different galaxies. Their strong winds are a major ingredient of
stellar evolution and population synthesis models. Yet, a coherent theoretical
picture of their strong mass-loss is only starting to emerge. In particular,
the occurrence of WR stars as a function of metallicity (Z) is still far from
being understood.
  To uncover the nature of the complex and dense winds of Wolf-Rayet stars, we
employ a new generation of model atmospheres including a consistent solution of
the wind hydrodynamics in an expanding non-LTE situation. With this technique,
we can dissect the ingredients driving the wind and predict the resulting
mass-loss for hydrogen-depleted massive stars. Our modelling efforts reveal a
complex picture with strong, non-linear dependencies on the luminosity-to-mass
ratio and Z with a steep, but not totally abrupt onset for WR-type winds in
helium stars. With our findings, we provide a theoretical motivation for a
population of helium stars at low Z, which cannot be detected via WR-type
spectral features. Our study of massive He-star atmosphere models yields the
very first mass-loss recipe derived from first principles in this regime.
Implementing our first findings in stellar evolution models, we demonstrate how
traditional approaches tend to overpredict WR-type mass loss in the young
Universe.
