RR Lyrae variables in the field and the Sandage period-shift effect in globular clusters

With the goal of obtaining reddening-free luminosity-to-mass ratio of ab-type RR Lyrae variables, the data of Lub (1977) for field RR Lyrae stars with well determined reddening are reanalyzed and the relationships among the period shift Delta log P(Te) and other observational parameters were determined. It is shown that there is a strong correlation among the period, the blue amplitude, and the light-to-mass ratio, which makes it possible to determine the light-to-mass ratio of any ab-type variable once its period and amplitude are known. The analysis of two additional samples of field variables yielded, for both samples, the slope of the relationship between the light-to-mass ratio and the metal content which is very near the slope for the Lub's stars (i.e., -0.10). In contrast, the slope for RR Lyrae variables in globular clusters was found to be -0.05, in close agreement with the prediciton of synthetic horizontal branch computations.