Sonic-point temperatures derived from a set of 73 M_⊙ stationary hydrodynamic stellar models, adopting several constant mass-loss rates. The sonic points located in the temperature range of the He II opacity bump are shown as solid black, while those located in the temperature range of the H/He I opacity bump are shown as solid gray. The two ranges are separated by the forbidden temperature range (Sect. 3). Superposed is the temperature-dependent mass-loss prescription adopted in our time-dependent calculations (dashed gold, see Sect. 4).