Anomalous RR Lyrae (V-I)(0) colors in Baade's window

We compare (V-I)(0)-(V-K)(0) color-color and (V-I)(0)-log P period-color di agrams for Baade's window and local RRab Lyrae stars. We iind that for a fi xed log P, the Baade's window RR Lyrae stars are similar to 0.17 mag redder in (V-I), than the local RR Lyrae stars. We also show that there is no suc h effect observed in (V-K)(0). We argue that an extinction misestimate towa rd Baade's window is not a plausible explanation of the discrepancy. Unlike Baade's window RR Lyrae stars, the local ones follow a blackbody color-col or relation and are well approximated by theoretical models. We test two pa rameters, iron metallicity and surface gravity, and find that their effects are too small to explain the (V-I)(0) discrepancy between the two groups o f stars. We suggest that the anomalous (V-I), behavior of the Baade's windo w RR Lyrae stars may be caused by an enhancement in alpha-element abundance . We note that a similar color discrepancy for clump giant stars has been r ecently reported by Paczynski, and we caution that RR Lyrae stars and clump giants, often used as standard candles, can be subject to the same type of systematics.