Variable Stars in Local Group Galaxies. I: Tracing the Early Chemical
  Enrichment and Radial Gradients in the Sculptor dSph with RR Lyrae Stars
  We identified and characterized the largest (536) RR Lyrae (RRL) sample in a
Milky Way dSph satellite (Sculptor) based on optical photometry data collected
over $\sim$24 years.
  The RRLs display a spread in V-magnitude ($\sim$0.35 mag) which appears
larger than photometric errors and the horizontal branch (HB) luminosity
evolution of a mono-metallic population. Using several calibrations of two
different reddening free and metal independent Period-Wesenheit relations we
provide a new distance estimate $\mu$=19.62 mag ($\sigma_{\mu}$=0.04 mag) that
agrees well with literature estimates. We constrained the metallicity
distribution of the old population, using the $M_I$ Period-Luminosity relation,
and we found that it ranges from -2.3 to -1.5 dex. The current estimate is
narrower than suggested by low and intermediate spectroscopy of RGBs
($\Delta$[Fe/H] $\le$ 1.5).
  We also investigated the HB morphology as a function of the galactocentric
distance. The HB in the innermost regions is dominated by red HB stars and by
RRLs, consistent with a more metal-rich population, while in the outermost
regions it is dominated by blue HB stars and RRLs typical of a metal-poor
population. Our results suggest that fast chemical evolution occurred in
Sculptor, and that the radial gradients were in place at an early epoch.
