Regions in the optical spectra used to define the CCF masks to study the chromaticity of the SOPHIE RVs. Upper panel: theoretical PHOENIX medium resolution models of stellar atmospheres with T_eff = 3400 and 3000 K in the SOPHIE wavelength range. Lower panel: Flux ratio of the atmosphere models of the upper panel. The horizontal red line is the median flux ratio. Lines of the SOPHIE M-star mask in the regions over the median flux ratio constitute the high-contrast mask, and the lines located below the median constitute the low-contrast mask.