RUPRECHT-55 OR THE 2-IN-ONE CLUSTER

The open cluster Ru 55 is found to consist of two partially superimpos ed groups of young stars, lying at 4.2+/-0.5 kpc, E((B-V))=0.60+/-0.15 mag (s.e.) (Ru 55A) and 7.5+/-1.1 kpc, E((B-V))=0.60+/-0.09 mag (s.e. ) (Ru 55B), respectively. Ru 55A has 27 members, 8 of them previously recognized as part of the former Ru 55 cluster; Ru 55B has 19 probable members, again with 8 objects from the old list. These groups overlap over a relatively small area, which includes the original open cluste r location. This eventually led to the idea, in previous papers, of it being a single object. Both clusters are very young, judging from the ir earliest photometric spectral types, which give an approximate age of 10(6) yr. Ru 55A is located in the Perseus arm or in the region whe re that arm and the Local one merge; and Ru 55B is in an external feat ure (+II), known as ''the 15 kpc arm.''