The Open Cluster Ruprecht 91 and Its Cepheids

Abstract

Photoelectric UBV photometry and star counts are presented for stars in the previously unstudied open cluster Ruprecht 91, supplemented by observations for stars in adjacent regions surrounding the Cepheids SX Car and VY Car, including new observations for the latter. Ruprecht 91 is typical of groups associated with Cepheids, with an evolutionary age of ~8 × 107 yr, but it is only 980 ± 8 pc distant, much closer than the Cepheids. Both Cepheids are unlikely to be members of Ruprecht 91: VY Car on the basis of location beyond the cluster tidal radius, age, and implied distance, and SX Car on the basis of implied distance. The brightest star in Ruprecht 91 is the M1 II supergiant HD 93662, a likely member. The suspected variability of the star is confirmed here with the aid of archival data, and its likely cycle length is found to be 5756 days (15.8 yr). Period changes in VY Car and SX Car are also studied with the aid of archival data. The period of VY Car is decreasing, its rate of -75.92 ± 0.37 s yr-1 being consistent with a second crossing of the instability strip. That for SX Car is increasing, its rate of +0.07 ± 0.02 s yr-1 being consistent with a third crossing. VY Car and SX Car are established to have space reddenings of EB-V = 0.18 ± 0.01 and 0.17 ± 0.01, respectively.