(From top to bottom) OHP/Mistral spectrum (blue) and best-matching synthetic one (red). The same OHP/Mistral data (blue) but with two other synthetic spectra calculated for different T_eff (5140 K green, and 5740 K – pink). The same OHP/Mistral data (blue) compared with two other synthetic spectra calculated for different log g (1.00 green, 4.50 – pink). The same OHP/Mistral data (blue) compared with two other synthetic spectra calculated for different [M/H] (0.00 – green, +0.50 – pink). All plots show the same spectral region around the Hα line.