Effective temperatures as a function of log  N iv/N iii, for O2-O4 sample stars. Spectral types distinguished according to legend. Upper panel: observed emission line ratios, limits indicated by arrows; lower panel: emission line ratios predicted from the (globally) best-fitting models. Objects numbered according to Table 6. Filling and symbols as in Fig. 12. Dashed vertical lines (at log  N iv/N iii = –1, 0, 1) correspond to the limits used in Fig. 6. For object #9 (NGC 346-355), T_eff and theoretical line ratio according to our “average” solution. Stars #4, #8, and #9 are SMC stars.