Modified wind momentum (D_mom) vs. luminosity diagram. The dashed lines indicate the theoretical predictions of Vink et al. (2001) for homogeneous winds. Top panel: the empirical fit for this work and Mokiem et al. (2007a) (both for L/L_⊙> 5.0) in shaded blue and gray bars, respectively. For stars with L/L_⊙ ≤ 5.0, only upper limits could be constrained. These stars are not considered in the analysis. Bottom panel: same as top panel but now for an analysis in which the acceleration of the wind flow, β, is a free parameter and in which the analysis does not include nitrogen lines, but relies on hydrogen and helium lines only.