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A solution to the old Oosterhoff controversy is proposed on the basis of anew theoretical pulsational scenario concerning RR Lyrae cluster variables(Bono and coworkers). We show that the observed constancy of the lowestpulsation period in both Oosterhoff type I (OoI) and Oosterhoff type II (OoII)prototypes (M3, M15) can be easily reproduced only by assuming the canonicalevolutionary horizontal-branch luminosity levels of these Galactic globularclusters and therefore by rejecting the Sandage period shift effect (SPSE).Comment: postscript file of 7 pages and 2 figures; one non postcript figure is  available upon request; for any problem please write to  marcella@astr1pi.difi.unipi.i

RR Lyrae Variable Stars: Pulsational Constraints Relevant to the Oosterhoff Controversy