Stromgren photometry in globular clusters: M55 & M22

We present Stromgren CCD photometry for the two galactic globular clusters M55 (NGC6809) and M22 (NGC6656). We find average Stromgren metallicities of -1.71+/-0.04 dex for M55 and -1. 62 +/- 0.08 dex for M22. The determination of metal abundances in cluster g iants with the Stromgren mi index in comparison with spectroscopic data fro m Briley et al. (1993) and Norris & Freeman (1982, 1983) shows that:M55 and M22 have different distributions of cyanogen strengths. In M55, no CN abun dance variations are visible among the giant-branch stars. In striking cont rast, a large dispersion of cyanogen strengths is seen in M22. For M22 we f ind patchily distributed variations in the foreground reddening of Delta E( B - V) approximate to 0.07, which explain the colour dispersion among the g iant-branch stars. There is no evidence for a spread in iron within M22 sin ce the variations in mi are dominated by the large range in CN abundances, as already found by Anthony-Twarog et al. (1995). The difference between M5 5 and M22 may resemble the difference in integral CN band strength between M31 globular clusters and the galactic system. The colour-magnitude diagram of M55 shows the presence of a population of 5 6 blue-straggler stars that are more centrally concentrated than the red gi ant-branch stars.