Double-mode RR Lyrae variables: Pulsational masses revisited

Double-mode RR Lyrae variables (i.e., radial variables which are simultaneously pulsating in both fundamental and first-overtone modes) appear to be fundamental tools for investigating the mass of old Population II horizontal-branch (HE) stars. The most widespread method adopted for evaluating the masses of these objects is based on the Petersen approach, which relies only on pulsational periods (P-1/P-0 versus P-0), and therefore is independent of any preliminary evaluation of the reddening and/or the distance modulus of the stellar cluster. In this paper we supply an overview of the mass estimates and underline the role played by opacities as well as by full-amplitude nonlinear models for removing the discrepancy between pulsational and evolutionary masses. On the basis of the comparison between the theoretical scenario and double-mode RR Lyrae stars belonging to selected Galactic globular clusters (IC 4499, M3, M15, M68, NGC 2419, NGC 6426), we show that the P-1/P-0 versus P-0 diagram can also provide valuable constraints on the luminosity of these variables.