Excerpt of Fig. 5 of Pauldrach et al. (2012) showing the appearance of the visible UV spectrum of massive O stars as a function of mass M for a constant effective temperature T_eff and radius R, resulting from dynamically consistent wind models. (T_eff = 40 000 K, R = 15 R_⊙, and solar abundances have been assumed for the models.) Although the mass of the first model is only 20% smaller than that of the second one, the resulting UV spectra appear markedly different. Because of this strong response, the UV spectrum can be used diagnostically for a determination of the stellar mass. A corresponding dependence of the appearance of the UV spectrum on the stellar parameters as shown here for massive O stars also holds for O-type CSPNs, which have comparable wind densities and velocities (see text), and thus similar UV spectra.