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We present an extensive survey for rapid pulsators among Extreme HorizontalBranch (EHB) stars in omega Cen. The observations performed consist of nearly100 hours of time-series photometry, as well as low-resolution spectroscopy. Weobtained photometry for some 300 EHB stars. Based on the spectroscopy, wederive reliable values of log g, Teff and log(N(He)/N(H)) for 38 targets, aswell as estimates of the effective temperature for another nine targets. Thesurvey uncovered a total of five rapid variables with multi-periodicoscillations between 85 and 125 s. Spectroscopically, they form a homogeneousgroup of hydrogen-rich subdwarf O stars clustered between 48,000 and 54,000 K.For each of the variables we are able to measure between two and threesignificant pulsations believed to constitute independent harmonicoscillations. In addition to the rapid variables, we found an EHB star with anapparently periodic luminosity variation of ~2700 s, which we tentativelysuggest may be caused by ellipsoidal variations in a close binary. Using theoverlapping photometry and spectroscopy sample we are able to map an empiricalomega Cen instability strip. This can be directly compared to the pulsationdriving predicted from the Montreal "second-generation" models. We find thatthe region where p-mode excitation occurs is bifurcated, and the well-knowninstability strip between 29,000-36,000 K where the rapid subdwarf B pulsatorsare found is complemented by a second one above 50,000 K. While significantchallenges remain at the quantitative level, we believe that the samekappa-mechanism that drives the pulsations in hot B subdwarfs is alsoresponsible for the excitation of the rapid oscillations observed in the omegaCen variables.

Pulsating hot O subdwarfs in Omega Cen: mapping a unique instability strip on the Extreme Horizontal Branch