Top and middle: comparison of the synthetic spectra of two best-fitting FASTWIND models to the observed spectrum of the O6.5 Vz star HD 199579 for three diagnostic lines. The first best-fitting model corresponds to the  IACOB-GBAT analysis with T_eff and log g as free parameters (the red part is fitted and the blue part is ignored). In the second model, log g was fixed to 4.2 dex. Bottom: χ² distributionsfor T_eff and log g resulting from the  IACOB-GBAT analysis. The horizontal dashed lines represent the value of χ² for the best-fitting model (red dots) and the 1σ and 2σ confidence levels. Any model with log g ~ 4.2 is clearly beyond the 2σ level.