Relative contribution of individual elements to the line radiative force at the wind critical point. Plotted as a function of the stellar effective temperature for models with M = 40 M_⊙. The thick blue line plots the mean ionization state that contributes to the line radiative force. This is defined as ∑f_iz_i∕(∑f_i), where z_i is the ionization state of ion i and f_i is its contribution to the line radiative force. This plot was derived in a simplified way using the Sobolev approximation.