  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.
