ON THE OOSTERHOFF-ARP-SANDAGE PERIOD-SHIFT EFFECT AND ITS EVOLUTIONARY INTERPRETATION

Synthetic models of the horizontal branch are employed to show that th e present theoretical predictions for the slope of the mean ab-type RR Lyrae period shift-metallicity relation, DELTA[log P(ab)] - [Fe/H], a re incompatible with the recent observational results by Sandage (1993 , AJ 106, 687), unless hysteresis is highly effective in the determina tion of RR Lyrae types. The new instability strip topology by Bono & S telling-werf (1992, MSAIt 63, 357) further strengthens this conclusion . However, the recent results of Catelan (1994, AJ, submitted) recomme nd caution before regarding hysteresis as the final explanation for ex isting problems involving RR Lyrae stars in Galactic globular clusters .