Empirical relations for cluster RR Lyrae stars revisited

Our former study on the empirical relations between the Fourier parameters of the light curves of the fundamental mode RR Lyrae stars and their basic stellar parameters has been extended to considerably larger data sets. The most significant contribution to the absolute magnitude MV comes from the period P and from the first Fourier amplitude A(1), but there are statistically significant contributions also from additional higher order components, most importantly from A(3) and in a lesser degree from the Fourier phase theta (51). When different colors are combined in reddening-free quantities, we obtain basically period luminosity color relations. Due to the log T-eff(B - V, log g; [Fe/H]) relation from stellar atmosphere models, we would expect some dependence also on theta (31). Unfortunately, the data are still not extensive and accurate enough to decipher clearly the small effect of this Fourier phase. However, with the aid of more accurate multicolor data on field variables, we show that this Fourier phase should be present either in V - I or in B - V or in both. From the standard deviations of the various regressions, an upper limit can be obtained on the overall inhomogeneity of the reddening in the individual clusters. This yields sigma (E)(B V) less than or similar to 0.012 mag, which also implies an average minimum observational error of sigma (V) less than or similar to 0.018 mag.