1999A&A...350..476R

We present Stroemgren CCD photometry for the two galactic globular clusters M 55 (NGC 6809) and M 22 (NGC 6656). We find average Stroemgren metallicities of -1.71±0.04dex for M 55 and -1.62±0.08dex for M 22. The determination of metal abundances in cluster giants with the Stroemgren m₁ index in comparison with spectroscopic data from Briley et al. (1993) and Norris & Freeman (1982, 1983) shows that M 55 and M 22 have different distributions of cyanogen strengths. In M 55, no CN abundance variations are visible among the giant-branch stars. In striking contrast, a large dispersion of cyanogen strengths is seen in M 22. For M 22 we find patchily distributed variations in the foreground reddening of ΔE(B-V)≃0.07, which explain the colour dispersion among the giant-branch stars. There is no evidence for a spread in iron within M 22 since the variations in m₁ are dominated by the large range in CN abundances, as already found by Anthony-Twarog et al. (1995PASP..107...32A). The difference between M 55 and M 22 may resemble the difference in integral CN band strength between M31 globular clusters and the galactic system. The colour-magnitude diagram of M 55 shows the presence of a population of 56 blue-straggler stars that are more centrally concentrated than the red giant-branch stars.