FASTWIND predictions for Vz behavior with log Q as a function of T_eff, for models with 3.8 ≤ log g ≤ 4.2 dex, and 50 ≤ v sin i ≤ 300 km s^-1, compared to the results from our analyses with the IACOB-GBAT. The shaded area indicates the region where we expect both O Vz and O V stars from our analysis (see text), whereas we expect only O V/O Vz stars above/below this area. Symbols as used in Fig. 7.