Top panels: I-b − y (left) and I-m₁ (right) CMD for M 22, corrected for differential reddening. The grey symbols represent HB stars (triangles), probable AGB stars (circles) and RGB bump stars (crosses) selected in the I-(b − y) CMD. Spectroscopic data are superimposed with s-rich stars represented by red circles, and s-poor stars by blue crosses, according with the other figures. Bottom panels: colour difference Δm₁ between each analyzed star and a reference fiducial line represented as a dotted line (left panel). On the right panel the  ⟨ [s/Fe] ⟩  average abundances are shown as a function of Δm₁. The sources for the photometric data are given in Sect. 2.