EVOLUTION OF HORIZONTAL-BRANCH STARS IN GLOBULAR-CLUSTERS - THE INTERESTING CASE OF V79 IN M3

New observations of variable stars in the globular cluster M3 reveal t hat the RR Lyrae variable V79 is a double-mode (RRd) variable with the first-overtone mode dominating. In all previous studies, V79 was foun d to be a fundamental-mode (RRab) pulsator with an irregular light cur ve. This is the first observed mode switch for an RR Lyrae variable, a nd it is direct observational evidence for blueward evolution of horiz ontal-branch stars in the Oosterhoff type I cluster M3. It also demons trates that there is a connection between the Blazhko effect and pulsa tional mode mixing in RR Lyrae variables, These new observations also show that the strength of the overtone oscillations in the RRd star V6 8 in M3 may have increased in the last 70 yr, thus indicating blueward evolution for V68 as well. A survey of previously published investiga tions of RRd stars in Oosterhoff type II systems indicates that there is marginal evidence for an increase in the strength of fundamental-mo de oscillations in two stars: V30 in M15 and AQ Lee. If these increase s are confirmed by future observations, it will indicate redward evolu tion for RRd stars in type II systems.