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Towards the end of their evolution hot massive stars develop strong stellarwinds and appear as emission line stars, such as WR stars or LBVs. Thequantitative description of the mass loss in these important pre-SN phases ishampered by unknowns such as clumping and porosity due to an in-homogeneouswind structure, and by an incomplete theoretical understanding of opticallythick stellar winds. In this work we investigate the conditions in deepatmospheric layers of WR stars to find out whether these comply with the theoryof optically thick winds, and whether we find indications of clumping in theselayers. We use a new semi-empirical method to determine sonic-point opticaldepths, densities, and temperatures for a large sample of WR stars of thecarbon (WC) and oxygen (WO) sequence. Based on an artificial model sequence weinvestigate the reliability of our method and its sensitivity to uncertaintiesin stellar parameters. We find that the WR stars in our sample obey anapproximate relation with P_rad/P_gas~80 at the sonic point. This 'windcondition' is ubiquitous for radiatively driven, optically thick winds, andsets constraints on possible wind/envelope solutions affecting radii, mass-lossrates, and clumping properties. Our results suggest that the presence of anoptically thick wind may force many stars near the Eddington limit to developclumped, radially extended sub-surface zones. The clumping in these zones ismost likely sustained by the non-linear strange-mode instability, and may bethe origin of the observed wind clumping. The properties of typical late-typeWC stars comply with this model. Solutions without sub-surface clumping andinflation are also possible but demand for compact stars with comparatively lowmass-loss rates. These objects may resemble the small group of WO stars withtheir exceptionally hot stellar temperatures and highly ionized winds.

Stellar mass-loss near the Eddington limit. Tracing the sub-photospheric layers of classical Wolf-Rayet stars