Galactic Clusters with Associated Cepheid Variables. V. The Case of SU CygNI.

Photoelectric, CCD, and photographic photometry is presented for a sample of 163 stars in the field of the short-period Cepheid SU Cyg for the purpose of searching for a putative cluster that might be associated with the Cepheid. Although the variable is situated in a clump of stars that has the appearance of a sparse cluster, the photometric evidence indicates that they are not a physical group. About two dozen B and A-type stars randomly distributed in the field lie at a common distance of l086±60 pc (V₀-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 radial velocity (<V_R>= - 23.6±7.1 km s^-1) close to the systemic velocity of the Cepheid, is a spectroscopic binary that might be physically related to the variable. Its primary importance is for helping to establish a space reddening of E_B-V=0.15±0.01 for SU Cyg. Despite the lack of related cluster stars to SU Cyg, its two unresolved companions can be used to place restrictions on the likely luminosity of the Cepheid. The resulting value of <M_v>≤-3.02±0.23 relative to its companions agrees closely with a value of <M_v>(PLC)=-3.03±0.07 obtained from the Cepheid PLC relation. 0 1997 American Astronomical Society.