A search for OB associations near Southern long-period Cepheids. IV. XY Carinae, GT Carinae, WZ Sagittarii, and SW Velorum

Abstract

We report UBV photometry for fields surrounding four long-period Cepheids in the Southern Milky Way. No potential early-type companions are detected around XY Car, which appears to be relatively isolated. However, an anonymous group of nine B-type and two possible FG supergiant stars are detected as possible companions to GT Car, despite its location at a seemingly improbably large distance of 10 kpc. WZ Sgr is found to lie in a sparse, previously undetected cluster, 1.8 kpc distant, containing upward of 76 potential early-type member stars. Preliminary spectroscopic observations strengthen the case for cluster membership in this instance. SW Vel appears to belong to an older subgroup of the 2.5 kpc distant Vel OB5 association described by Turner (1979), with 28 B star candidates in its immediate vicinity. Derived space reddenings and luminosities for WZ Sgr and SW Vel are consistent with their group membership, and the parameters of their associated stars, as well as those for the potential companions of GT Car, indicate that they are of comparable age to their associated Cepheid. Information is provided for follow-up studies of these stellar groups.