Residuals of the χ_red²-minimization in the mass-metallicity plane for both components (primary left, secondary right) for identical metallicities for both components (Case D1). For this case, the magenta triangle marks the best-fitting solution. For the analogous case, with both model metallicities forced to be in the 1-σ-uncertainty of the spectroscopic value (Case D3), the best fitting solution is given as a blue cross. The dashed lines show the 1σ-errorbar around the spectroscopic metallicity.