1999ApJ...521..206S

We compare (V-I)₀-(V-K)₀ color-color and (V-I)₀-log P period-color diagrams for Baade's window and local RRab Lyrae stars. We find that for a fixed log P, the Baade's window RR Lyrae stars are ∼0.17 mag redder in (V-I)₀ than the local RR Lyrae stars. We also show that there is no such effect observed in (V-K)₀. We argue that an extinction misestimate toward 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-color relation and are well approximated by theoretical models. We test two parameters, iron metallicity and surface gravity, and find that their effects are too small to explain the (V-I)₀ discrepancy between the two groups of stars. We suggest that the anomalous (V-I)₀ behavior of the Baade's window RR Lyrae stars may be caused by an enhancement in α-element abundance. We note that a similar color discrepancy for clump giant stars has been recently reported by Paczyn'ski, and we caution that RR Lyrae stars and clump giants, often used as standard candles, can be subject to the same type of systematics.