Cepheids in Magellanic Cloud star clusters: Fundamental and overtone pulsators in NGC 2157

As part of a systematic CCD survey to search for and study Cepheids in young Magellanic Cloud star clusters, we report the discovery of three Cepheids in the Large Magellanic Cloud star cluster NGC 2157. Two of these variables have periods of approximately 3 days, while the third has a period of ∼7.7 days. The photometric properties of the two short-period Cepheids are consistent with their being overtone pulsators, while the long-period Cepheid is more likely pulsating in the fundamental mode. We reach the same conclusion by calculating the pulsational masses for the new Cepheids using a consistent set of theoretical relations. The evolutionary/pulsational mass discrepancy known for Galactic Cepheids reappears for these extragalactic Cepheids with our assumption that the short-period NGC 2157 variables are overtone pulsators. Mass loss can account for neither the observed period range nor the mass discrepancy unless extremely large and unprecedented mass-loss rates are assumed. Stellar models incorporating modified treatments of convection may provide a solution for this new version of the Cepheid mass discrepancy. Our observations of the NGC 2157 Cepheids along with recent results for other Magellanic Cloud cluster Cepheids suggest that the fraction of short-period (≲ 7 days) Cepheids that are first overtone pulsators may be much larger than is generally believed.