RR Lyrae variable stars in the dwarf spheroidal galaxy Leo I

We report the discovery of a significant population of RR Lyrae variable st ars 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 galax y. Full coverage of the light variations and pulsation periods have been ob tained 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 min imum period of 0.54 days. The pulsational properties indicate an intermedia te 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 v ariable stars, and the significant number of RRab stars with long period an d large amplitude, suggest that the bulk of the old population in Leo I is more like the Oosterhoff type II globular clusters. The most straightforwar d interpretation is that a range in metallicity is present among the RR Lyr ae stars of Leo I, with a significant population of very metal-poor stars. Alternatively, these Oosterhoff type II variable stars could be more evolve d. The average apparent magnitude of the RR Lyrae stars across the full cyc le 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.