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M22 is an anomalous globular cluster that hosts two groups of stars withdifferent metallicity and s-element abundance. The star-to-star light-elementvariations in both groups, with the presence of individual Na-O and C-Nanticorrelations, demonstrates that this Milky-Way satellite has experienced acomplex star-formation history. We have analysed FLAMES/UVES spectra for sevenstars covering a small color interval, on the reddest horizontal-branch (HB)portion of this cluster and investigated possible relations between thechemical composition of a star and its location along the HB. Our chemicalabundance analysis takes into account effects introduced by deviations from thelocal-thermodynamic equilibrium (NLTE effects), that are significant for themeasured spectral lines in the atmospheric parameters range spanned by ourstars. We find that all the analysed stars are barium-poor and sodium-poor,thus supporting the idea that the position of a star along the HB is strictlyrelated to the chemical composition, and that the HB-morphology is influencedby the presence of different stellar populations.

Horizontal-branch morphology and multiple stellar populations in the anomalous globular cluster M22