Results of the analysis of synthetic spectra from FASTWIND models. Six parameter combinations: T_eff–log g = 32 kK–3.5 dex, 35 kK–3.5 dex, and 40 kK–3.9 dex (red, green, and blue symbols, respectively), each with Y_He–ϵ_N = 0.10–7.8 dex and 0.15–8.6 dex (filled and open symbols, respectively). For each model, we show the input values (circles) and the results obtained from the synthetic spectra with 0%, 5%, and 10% continuum dilution (squares, stars, and triangles, respectively). Top: Spectroscopic HR diagram. For reference, we include the non-rotating evolutionary tracks from Ekström et al. (2012). Dotted lines indicate constant gravities, starting at log g = 4.2 dex on the left and decreasing in steps of 0.2 dex. Bottom: Nitrogen abundance versus He abundance (as in Fig. 6). For reference, we include GENEC tracks for initial masses between 20 and 60 M_⊙ and v_ini/v_crit  =  0.2 − 0.4.