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We analyze the reddening, surface helium abundance and mass of 115 horizontalbranch (HB) and blue hook (BH) stars in OmegaCentauri, spanning the HB from theblue edge of the instability strip to Teff~50000K. The mean cluster reddeningis E(B-V)=0.115+-0.004, in good agreement with previous estimates, but weevidence a pattern of differential reddening in the cluster area. The stars inthe western half are more reddened than in the southwest quadrant by 0.03-0.04magnitudes. We find that the helium abundances measured on low-resolutionspectra are systematically lower by ~0.25 dex than the measurements based onhigher resolution. No difference in helium abundance is detected betweenOmegaCentauri and three comparison clusters, and the stars in the range11500-20000K follow a trend with temperature, which probably reflects avariable efficiency of the diffusion processes. There is mild evidence that twofamilies of extreme HB (EHB) stars (Teff>20000K) could exist, as observed inthe field, with ~15% of the objects being helium depleted by a factor of tenwith respect to the main population. The distribution of helium abundance above30000K is bimodal, but we detect a fraction of He-poor objects lower thanprevious investigations. The observations are consistent with these being starsevolving off the HB. Their spatial distribution is not uniform, but thisasymmetric distribution is only marginally significative. We also find that EHBstars with anomalously high spectroscopic mass could be present inOmegaCentauri, as previously found in other clusters. The derivedtemperature-color relation reveals that stars hotter than 11000K are fainterthan the expectations of the canonical models in the U band, while no anomalyis detected in B and V. This behavior, not observed in NGC6752, is a newpeculiarity of OmegaCentauri HB stars.

Spectroscopy of horizontal branch stars in Omega Centauri