The modified wind momentum as a function of stellar luminosity using the data of Mokiem et al. (2007). Black symbols refer to mass-loss estimates based on the fitting of the Hα-profile; grey symbols are mass-loss estimates that rely strongly on ultraviolet resonance lines. Note that the Hα estimates at L < 10^5.2   L_⊙ are upper limits. A steep jump – of about 2 dex – can be seen at a luminosity of 10^5.2 L_⊙. The red dots are our predictions for D_mom. The squares denote the supergiants, the circles the giants and the triangles the main-sequence stars. Below 10^5.2 L_⊙ we do not find wind solutions for dwarf stars. At higher luminosity our predictions are close to the observed values. We therefore interpret the origin of the weak-wind problem in dwarf stars to be connected to a lack of line driving for objects less bright than about 10^5.2 L_⊙.