Demonstration of FeH line strength dependency on the effective temperature and overall metallicity. The contour plots in this figure show the calculated χ² of the fit based on a grid of metallicity and effective temperature of all M dwarfs. The block lines and dots indicate the temperature with the minimum χ² for each step in metallicity. The plots are shown in order of spectral type, starting from the coolest going towards the warmest target in our sample. For the eight coolest stars, the χ² value strongly depends on the effective temperature, with only a weak dependency on the metallicity. For the warmer stars (above 3400 K) a slight dependency on effective temperature can be seen, and for the warmest M dwarf HIP 57172 B (M0-M1) in our sample, a clear degeneracy between metallicity and effective temperature is found and this method cannot be used to determine the effective temperature for this target.