Period changes in omega Centauri RR Lyrae stars

A century of observations of omega 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 d ominate 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 RRa b 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 s how a much more complex, irregular behavior.