Observed HARPS-N spectrum of the SB2 system S 492 (black dots) around the Mg I b lines (λ 5160 Å, upper panel) and around λ 6440 Å (lower panel). In each panel the synthetic spectrum, which is the weighted sum of two standard star spectra mimicking the primary and secondary component of S 492, is overplotted with a full red line. The insets show the χ² of the fit as a function of the contribution of the primary spectrum, w^P.