Variable Stars in (Not Only) Dwarf Galaxies: Key Tools to Constrain Distances and Stellar Content

The important role of Cepheid and RR Lyrae variable stars and what they teach us about dwarf galaxies is discussed. Despite ever improving star formation histories of Local Group dwarf galaxies uncertainties remain, in particular in the identification and characterisation of the oldest stellar populations. The old stellar populations sometimes can be hard to interpret, or even to detect, due to their inherent faintness and scarcity, and often a strong overlying young population makes the crowding due to much brighter stars difficult to overcome. Recent and some preliminary results for RR Lyrae searches (in M 32 and Leo A) carried out with HST and Gemini-North/GMOS are presented. In these cases variables represent the only way to constrain the nature and, sometimes, the presence of a stellar population >= 10 Gyrs old. The recent discovery of Cepheids in I Zw 18 with HST is discussed. This allowed the first accurate distance determination, enabling a more secure detection of the Tip of the Red Giant Branch, and confirmed the existence of a population >= 2 Gyrs old