Metallicity distributions for the samples considered. Upper panel: normalized distributions of 12 + Log(O/H) for ringed galaxies (solid red line) and for the control sample (long-dash black line). Lower left panel: galaxies with an inner ring (dashed magenta line) and galaxies with other types of rings (two-dash blue line). Lower right panel: non-barred ringed galaxies (dashed cyan line) and barred ringed galaxies (two-dashed green line).