  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.
