CCD BV photometry in M55

Combining these new data with previously published photographic photometry of cluster giants and horizontal-branch stars yields a high-quality composite color-magnitude diagram (CMD). A comparison of the latter with recent CMDs for M68 and NGC 6752-two clusters whose metallicities bracket that of M55-suggests (1) that the reddening to M55 may be as high as E(B-V)=0.14mag, and (2) that all three systems have similar ages.

In particular, if the apparent distance modulus to M55 is (m-M)_V_=14.10mag, then isochrones for Y=0.24 and high [O/Fe] would suggest an age of ~14Gyr. Very encouraging agreement of the observations with theoretical predictions is found.