Cr ii 5237 Å (left) and Fe ii 5427 Å (middle) lines of the primary, and Fe i 4476 Å (right) line of the secondary, for the 19 observations described in the text (Sect. 3.3.1, overplotted coloured lines). The mean (diamonds) and standard deviations (vertical bars) of each pixel inside and around the line have been overplotted and shifted by −0.05 in the y direction for the left and right panels, and by −0.02 for the middle panel. Red diamonds represent the pixels inside the spectral line, while blue diamonds represent the pixels in the local continuum. The averages of the standard deviations inside the line (σ(line)) and in the continuum (σ(cont)) are indicated in the bottom left corners.