Abstract

We present new spectroscopic metal abundances for 74 RR Lyrae (RRL) stars in ? Cen obtained with FLAMES. The well-known metallicity spread is visible among the RRL variables. The metal-intermediate (MI) RRL stars ([Fe/H] ? -1.2) are fainter than the bulk of the dominant metal-poor population ([Fe/H] ? -1.7), in good agreement with the corresponding zero-age horizontal-branch models with cosmological helium abundance Y = 0.246. This result conflicts with the hypothesis that the progenitors of the MI RRL stars correspond to the anomalous blue main-sequence stars, which share a similar metallicity but whose properties are currently explained by assuming for them a large helium enhancement. Therefore, in this scenario, the coexistence within the cluster of two different populations with similar metallicities ([Fe/H] ? -1.2) and different helium abundances has to be considered. © 2006. The American Astronomical Society. All rights reserved.