Spectral synthesis examples. The panels show the C₂ molecular band regions at 5128 Å (left) and 5165 Å (middle), and the C I atomic line at 5380.3 Å (right) for the HARPS spectrum of K2-24 (S/N ~ 50 at 5000 Å, T_eff = 5768 K, [Fe/H] = 0.41). The blue dots represent the observed spectrum, and the coloured lines are the synthetic spectra fitted to the data for models with different C abundances, separated by 0.1 dex. The bottom panels show the differences between observed and computed spectra. The shaded regions indicate wavelengths not used in the fit.