RR Lyrae variable stars in the dwarf spheroidal galaxy Leo I

We report the discovery of a significant population of RR Lyrae variable stars in the dwarf spheroidal galaxy Leo I. Based on 40 V and 22 B images of the galaxy taken using the ESO Wide-Field Imager, we have so far identified 74 candidate RR Lyrae stars in two CCDs hosting the main body of the galaxy. Full coverage of the light variations and pulsation periods have been obtained for 54 of them, 47 of which are Bailey ab-type RR Lyrae (RRab) stars and seven are c-type RR Lyrae (RRc) stars. The period distribution of the presently confirmed sample of RRab stars peaks at P = 0.60 days, with a minimum period of 0.54 days. The pulsational properties indicate an intermediate Oosterhoff type for Leo I, similar to other dwarf galaxies in the Local Group and the LMC. However, the rather long minimum period of the ab-type variable stars, and the significant number of RRab stars with long period and large amplitude, suggest that the bulk of the old population in Leo I is more like the Oosterhoff type II globular clusters. The most straightforward interpretation is that a range in metallicity is present among the RR Lyrae stars of Leo I, with a significant population of very metal-poor stars. Alternatively, these Oosterhoff type II variable stars could be more evolved. The average apparent magnitude of the RR Lyrae stars across the full cycle is ([V(RR)] = 22.60 +/- 0.12 mag, yielding a distance modulus (m-M)(v,o) = 22.04 +/- 0.14 mag for Leo I on the "long" distance scale.