Illustration of the high temperature and magnetic sensitivity of FeH lines which is reproduced by our models. Observed (black) and synthetic (red) intensity and circular polarization profiles for FeH at 9897–9913 Å in the considered sample of M dwarfs. The synthetic profiles were calculated using two components: one non-magnetic (“photospheric”) and one magnetic (“spot”) atmosphere model (Allard et al. 2001) with best-fit parameters obtained from fits of Stokes I and V spectra for FeH in the considered wavelength region only. All the following parameters were obtained simultaneously in one χ² minimization: the photosphere and spot temperatures, the magnetic field strength, the spot filling factor for the intensity f_I, and the circular polarization f_V.