Convective pulsating models with Y=0.24, stellar mass M=0.65Mo, 0.75Mo, under selected assumptions about luminosities and effective temperatures, are used together with stellar atmosphere computations to get the predicted dependence of RR Lyrae instability strip on stellar mass in the observative plane MV, B-V. The comparison with observed variables belonging to the globular clusters M3 and M15 is presented. It appears that in M3 (Oosterhoff type I) the transition between ab-type and c-type RR Lyrae takes place along the fundamental blue edge, whereas in M15 (Oosterhoff type II) it occurs near the first overtone red edge. This result supports the key role of the hysteresis mechanism originally proposed by van Albada & Baker for explaining the Oosterhoff dichotomy.

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