Conditions at the sonic point for a 14 M_⊙ helium star with Galactic metallicity. The solid and dashed black lines indicate stellar envelope solutions with different clumping factors D = 1 and 16 in the sub-surface layers. The conditions imposed by the optically thick stellar wind are indicated in blue. Labels indicate the stellar temperature T_⋆ of the wind solutions. The red dotted line indicates 50% of the photon-tiring limit, i.e. the region where half of the stellar luminosity L is consumed to drive the stellar wind.