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 a fundamental tool for investigating the mass of old Population II Horizontal Branch (HB) stars. The most widespread method adopted for evaluating the masses of these objects is based on the Petersen (1973) approach, which relies only on pulsational periods and therefore is independent of any preliminary evaluation of the reddening and/or of 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 (IC4499, M3, M15, M68, NGC2419, NGC6426) we show that the Petersen diagram can provide valuable constraints also on the luminosity of these variables. 
Giuseppe Bono, Filippina Caputo, Vittorio Castellani and Marcella   Marconi