  We present a simple analytical method to describe the structure of a
spherically expanding envelope with strong mass outflow. The structure is
consistently connected to the hydrostatic stellar interior and provides an
adequate description of the outer boundary conditions for stellar models with
large mass loss rates. We apply our treatment to evolutionary models of
Wolf-Rayet (WR) stars in order to study the possible influence of the stellar
winds on the interior, and to determine more reliable radii of WR stars. The
interior structure and evolution of WR stars is found to be unaffected by the
outer layers. For hydrogen rich WR stars (WNL) we find the existence of a
temperature domain in the HR-diagram, where a transient concentration of stars
on their blueward track is predicted in case of a strong backwarming from the
wind. For WNE and WC/WO stars we also derive subphotospheric radii
corresponding to Rosseland optical depths of ~ 10-20. With respect to wind-free
stellar models the subphotospheric radii are increased by up to a factor of ~ 4
for the most luminous WNE or WC stars. These radii and the corresponding
effective temperatures should roughly be comparable with the stellar parameters
of non-LTE atmosphere models of WR stars. Comparisons using the newly derived
radii yield a better agreement with observations. The stellar parameters
obtained with the new treatment allow a better assignment of theoretical
spectra to evolutionary tracks of evolved WR stars. This also provides the base
for future studies of the spectral evolution of post-MS massive stars and their
descendants. We also point out the possible importance of the iron opacity peak
for the acceleration of WR winds in the optically thick part, which may be
essential for the understanding of the dynamics of WR winds.
