How good are RR Lyrae and Cepheids really as Distance Indicators? The observational approach

Abstract

A number of recent technical developments, including the Hipparcos satellite, the Hubble Space Telescope fine guidance sensors and long base line near-IR interferometry has made it possible to employ several largely geometrical methods to determine direct distances to RR Lyrae stars and Cepheids. The distance scale now rests on a much firmer basis and the significant differences between the distances based on RR Lyrae stars (short) and Cepheids (long) to the LMC have been largely eliminated. The effects of metalicity on the RR Lyrae period-luminosity (PL) relation in the K-band as well as on the Cepheid PL relation appears to be the main remaining issues but even here empirical results are beginning to show convergence. I review here some of these recent developments seen from the perspective of the near-IR surface brightness method.