RR Lyrae the Stellar Beacons of the Galactic Structure
  We present some recent findings concerning the use of RR Lyrae as distance
indicators and stellar tracers. We outline pros and cons of field and cluster
RR Lyrae stars and discuss recent theoretical findings concerning the use of
the Bailey (amplitude vs pulsation period) diagram to constrain the possible
occurrence of Helium enhanced RR Lyrae stars. Nonlinear, convective RR Lyrae
models indicate that the pulsation properties of RR Lyrae stars are minimally
affected by the helium content. The main difference between canonical and He
enhanced models is due to the increase in luminosity predicted by evolutionary
models. Moreover, we focus our attention on the near-infrared Period-Luminosity
(PL) relation of RR Lyrae and summarize observational evidence concerning the
slope of the K-band PL relation in a few globulars (M92, Reticulum, M5, Omega
Cen) covering a range in metallicity of ~1 dex. Current findings suggest that
the slope has a mild dependence on the metal content when moving from the
metal-poor to the metal-intermediate regime.
  Finally, we also discuss the use of RR Lyrae stars either to estimate (helium
indicator: A-parameter) or to measure (absorption and emission lines) the
helium content.
