RR Lyrae variable stars

RR Lyrae's have long been recognized to be excellent tracers of old stellar populations, as well as primary distance indicators for Population II systems. Being primary distance indicators in the Galaxy and in the Magellanic Clouds, RR Lyr's are cornerstones of cosmological distance and time scales.

The absolute magnitude of the RR Lyrae's M$_{\rm V}$(RR) is known to depend on metallicity: M$_{\rm V}$(RR)=$\alpha \times$ [Fe/H] + $\beta$. However, there is no agreement either on the zero-point $\beta$ or on the slope $\alpha$ of this relation, since $\beta$ $\sim$ 0.5 or 0.75 mag (at [Fe/H]=$-$1.5) in the long and short distance scales, respectively, and literature values for $\alpha$ range from 0.30 mag/dex to 0.18-0.20 mag/dex.

RR Lyrae stars and their role in establishing the astronomical distance scale have been a major field of study at the OAB since 1984 (e.g. Cacciari et al. 1987). At present, several research programs at various levels of completion are being carried out.