CH(λ4300) and S(λ3839) band-strength index versus extinction-corrected apparent visual magnitude for probable member Sculptor RGB stars in four metallicity bins. Black asterisks denote objects belonging to the kinematic substructure (see text). The range of metallicities considered for each bin is given in the upper part of each panel. We report the mean error associated with index measurements in the top left corner of each panel. Solid lines are best fits to the data when the two notable S(λ3839) outliers in the most metal-poor bin are excluded. The slope of the best-fit line (a) and its associated error (σ) are also reported in the upper right corner of each panel. Error bars for the stars with strong CN absorption are explicitly given in the last panel.