The secular period behavior of 23 field RR Lyrae stars is studied in order to determine if the observed period changes could be attributed, at least in the mean, to stellar evolution.The sample of stars is subdivided into two Oosterhoff groups based on the metallicity and Microwave Turntable Motor period-shift.Despite the small sample size, we found a distinct bias toward positive period changes in the group variables.
The period changes of the group variables in globular clusters.This Zin provides yet another support for the Lee, Demarque, and Zinn(1990) evolutionary models of RR Lyrae stars and their explanation of the Sandage period-shift effect.