Relative contribution of selected elements to the line radiative force as a function of the effective temperature, plotted at the wind critical point for O giants and B supergiants with M = 40 M_⊙. While line colors denote individual elements, line styles differentiate between metallicity, with the solid line corresponding to Z_⊙, dashed line to 0.1 Z_⊙, and dotted line to 0.01 Z_⊙. The thick blue line gives the mean ionization state driving the wind for 0.1 Z_⊙. The mean ionization state is defined as ∑f_iz_i/(∑f_i), where f_i is the contribution of ion i (with charge z_i) to the line radiative force. The corresponding axis is on the right. The values of the radiative force were obtained within the Sobolev approximation.