GALACTIC CLUSTERS WITH ASSOCIATED CEPHEID VARIABLES .5. THE CASE OF SU CYGNI

Photoelectric, CCD, and photographic photometry is presented for a sam ple of 163 stars in the field of the short-period Cepheid SU Cyg for t he purpose of searching for a putative cluster that might be associate d with the Cepheid. Although the variable is situated in a clump of st ars that has the appearance of a sparse cluster, the photometric evide nce indicates that they are not a physical group. About two dozen B an d A-type stars randomly distributed in the field lie at a common dista nce of 1086+/-60 pc (V-0-M-V=10.18+/-0.12 for R=2.94+/-0.38), but they are likely to represent at best a segment of the Vul OB4 association, which overlaps the region. SU Cyg is unlikely to be a member of that group. The close optical companion to SU Cyg, an A2 V star with a radi al velocity ([V-R]=-23.6+/-7.1 km s(-1)) close to the systemic velocit y of the Cepheid, is a spectroscopic binary that might be physically r elated to the variable. Its primary importance is for helping to estab lish a space reddening of EB-V=0.15+/-0.01 for SU Cyg, Despite the lac k of related cluster stars to SU Cyg, its two unresolved companions ca n be used to place restrictions on the likely luminosity of the Cephei d, The resulting value of [M-V]less than or equal to-3.02+/-0.23 relat ive to its companions agrees closely with a value of [M-V](PLC)=-3.03/-0.07 obtained from the Cepheid PLC relation. (C) 1997 American Astro nomical Society.