Top: Example of the EW ratio of Si III-IV used as T_eff indicator with FASTWIND models for the star J20272428+4115458. Black, gray, blue, green and purple lines correspond to log g values of 3.9, 4.0, 4.1, 4.2 and 4.3 dex respectively. The horizontal dashed line indicates the EW ratio of the star and its associated uncertainty is indicated by the dash-dotted lines. Bottom: Example of the H Balmer lines fitting to determine log g for the same star. Only two FASTWIND models are compared with the observed spectrum (black line) for clarity. In red, the T_eff = 30 000 K, log g = 3.9 dex model. In green, the T_eff = 31 000 K, log g = 4.2 dex model. Final fits can be seen in Figs. C.1–C.8.