Minimum luminosity-to-mass ratio (in units of L_⊙ / M_⊙ , left vertical axis) and corresponding luminosity according to the mass–luminosity relation from Langer (1989, right vertical axis), as a function of mass-loss rate (in M_⊙ yr ⁻¹) for an outflow to be driven to supersonic velocities by the radiative acceleration from the iron opacity bump. The green symbols indicate H-free WNE stars from Hamann et al. (2006), with the different shapes indicating different spectral subclasses.