RR LYRAE VARIABLES IN THE FIELD AND THE SANDAGE PERIOD-SHIFT EFFECT IN GLOBULAR-CLUSTERS

The argument of this paper is the determination of a reddening-free way to obtain the luminosity-to-mass ratio of ab-type RR Lyrae variables. For such a purpose, Lub's data for field RR Lyrae stars with well-determined reddening are.reanalyzed and relationships among the relevant observational parameters are derived. In particular, we obtain tight period-amplitude-metallicity and period-intrinsic color-metallicity relations. Furthermore, it is shown that a strong correlation exists among period, blue amplitude, and light-to-mass ratios. This relation should permit the light-to-mass ratio of any ab-type variable to be determined once its period and amplitude are known. On these grounds, two additional samples of field variables are analyzed. For both samples the resulting slope of the relationship between the light-to-mass ratio and metal content is very near -0.10, which is that found for the Lub's stars. On the contrary, when RR Lyrae variables in globular clusters are analyzed, then the slope tums out to be approximately -0.05. Such a slope, which is much milder than that derived by Sandage for the same clusters, is in close agreement with the predictions of synthetic horizontal branch computations, and confirms the Lee suggestion that the Lub's sample contains many metal-poor variables highly evolved from the blue side of the instability strip.