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Published online by Cambridge University Press: 14 August 2015
UBV observations of stars in the small and compact cluster Tr 15 located 20 arcmin north of Eta Carinae confirms its physical connection to the OB aggregate comprising TR 14, Tr 16 and Cr 228. Its corrected distance modulus is Vo–Mv = 12.1, with a mean interstellar absorption of Av = 1.4 ± 0.2 mag. Two very red variable stars, RT Car, close to the cluster (2 arcmin), and BO Car, about 8 arcmin southeast, appear to be related to the cluster itself. These two are the only bright red supergiants in the whole region, and from the RICK measures the probable membership of both is verified. The extinction law from the B-V, V-I diagram indicates a higher value of R in comparison with the normal one, but lower than the amount derived earlier for the Tr 14/16 (Feinstein, Marraco, and Muzzio 1973), and which is also confirmed with new measures in this photometric system. The extinction law for the young cluster NGC 6611 (Feinstein, in preparation) appears to be the same, in spite of being probably younger.