Exploring the Leo II dSph I.: The Variable Star Content

We present the first comprehensive catalogue 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 data. We have also confirmed the existence of four anomalous Cepheids as identified in previous studies. The average period of the RR Lyrae ab variables (0.62 days), the fraction of c variables (0.24) and the minimum period of 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 derive a metallicity for Leo II of approximately [Fe/H]=-1.9. The presence of longer period, higher amplitude RR Lyrae variable 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 ``Ootershoff intermediate'' objects, falls between the Oosterhoff Class I and Oosterhoff Class II clusters. The properties of the variable populations of these objects are consistent with the idea that the Oosterhoff "dichotomy" is a continuum. The gap between the classes seems to be explained by the horizontal branch of Galactic globular clusters shifting away from the instability strip at at intermediate metallicities. However, Leo II, as well as other Oosterhoff intermediate objects, has a second parameter effect strong enough to leave horizontal branch stars in the instability strip.