Multiperiodic RR Lyrae Stars in Omega Centauri
  We have conducted a systematic search for multiperiodic pulsations in the RR Lyrae-type stars of the galactic globular cluster Omega Cen. Secondary periodicities close to the primary pulsation frequency have been detected in 17 out of 70 studied fundamental mode (RRab) pulsators and in 35 out of 81 overtone (RRc) pulsators. Because of the observed period ratios, these newly detected periodicities must correspond to nonradial modes. Their beating with the primary radial pulsation leads to a slow amplitude and phase modulation, commonly referred to as the Blazhko effect. The incidence rate of Blazhko modulation in Omega Cen RRab stars (24 +/- 5%) is similar to that observed in the Galactic Bulge. In the case of Omega Cen RRc stars, the incidence rate of Blazhko effect is exceptionally high (38 +/- 5%), more than 3 times higher than in any other studied population.   In addition to Blazhko variables, we have also identified two RR Lyr variables exhibiting first overtone/second overtone double-mode pulsations, and a triple-mode High Amplitude Delta Scuti variable. 
P. Moskalik and A. Olech