Period Changes in ω Centauri RR Lyrae Stars

A century of observations of ω Centauri allows us to accurately follow the period changes of a large sample of variable stars. Although the period changes are composed of monotone and irregular changes, period increases dominate in most of the RRab stars. The typical period increase rates are of the same order of magnitude as model calculations predict for the evolved, redward phase of horizontal-branch evolution. For the 44 well-observed RRab stars that show monotonic period changes, a mean rate of 0.15 days Myr -1 has been found. The period changes of the first-overtone RRc-type stars show a much more complex, irregular behavior.