Nonlinear models of RR Lyrae variables were calculated in the region of the HR diagram containing double mode pulsation. In all models Teff = 7000 K, the luminosity was 60 times solar, X = 0.70, Y = 0.299, and P1/P0 = 0.746, characteristic of double mode RR Lyrae variables in M15 (Cox, Hodson, and Clancy, 1983). Models of 0.65 solar masses were calculated using standard opacities and models of 0.75 solar masses were calculated with opacities artificially enhanced by a factor of 1.2 between log(T) = 5.2 and log(T) = 5.9 (Andreasen and Petersen, 1988).