NGC 2204: an old open cluster in the halo.

Photographic BV and photoelectric UBV photometry of stars in the open cluster NGC 2204 is presented. The cluster has a long giant branch reaching (B-V)0 I 8 which probably includes a red variable of substantial amplitude. The giants appear to have a range in colour reminiscent of their counterparts in NGC i88. There is a prominent giant clump at V = 13rn.82 and the upper main sequence has a gap about 0m.4 below its brightest point. Several blue stragglers are present, strongly concentrated towards the cluster centre. The reddening E(B-V) = orn.o8 and an ultraviolet excess (U-B)0.6 = 0m.095 + 0m.024 are derived from the two-colour diagram of probable cluster members. The cluster appears to be distinctly poor in heavy elements. The turn-off colour indicates an age of about 3 x io yr and the adopted distance modulus, (m-M)o = 13m.25 + 0m.2 (estimated uncertainty) implies that the cluster lies about 1250 pc from the galactic plane, an exceptional distance for a Population I system.