Exploring the Leo II dwarf spheroidal galaxy. I. The variable star content

We present the first comprehensive catalog of variable stars in the Leo II dwarf spheroidal galaxy. We have identified 148 RR Lyrae type variables, of which 140 were amenable to derivation of variability parameters with our d ata. We have also confirmed the existence of four anomalous Cepheids as ide ntified in previous studies. The average period of the RR Lyrae ab variable s (0.62 days), the fraction of c variables (0.24), and the minimum period o f the RR Lyrae ab variables (0.51 days) all define Leo II as an "Oosterhoff intermediate" galaxy. We have used the properties of these variables to de rive a metallicity for Leo II of approximately [Fe/H] = - 1.9. We attempt t o resolve discrepancies between this value and those determined by previous efforts. The presence of longer period, higher amplitude RR Lyrae variable s implies a metallicity distribution that extends to as poor as [Fe/H] = -2 .3. Leo II's location on the period-metallicity relation of clusters, like that of other "Oosterhoff intermediate" objects, falls between the Oosterho ff class I and Oosterhoff class II clusters. The properties of the variable populations of these objects are consistent with the idea that the Oosterh off "dichotomy" is a continuum. The gap between the classes seems to be exp lained by the horizontal branch of Galactic globular clusters shifting away from the instability strip at intermediate metallicities. However, Leo II, as well as other Oosterhoff intermediate objects, has a second-parameter e ffect strong enough to leave horizontal-branch stars in the instability str ip.