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The Astronomical Journal


We present an analysis of the period-V-amplitude plane for RR0 Lyrae stars (fundamental mode pulsators) with "normal" light curves in the bulge using the MACHO bulge fields. Although bulge globular clusters (GCs) have RR Lyraes that divide into two reasonable distinct groups according to the average period of the RR0 Lyraes, there is no evidence of a gap between Oosterhoff I (OoI) and II (OoII) stars in the bulge field star sample. The majority of the bulge RR0 Lyrae field star population have a difference in period compared to the OoI cluster M3 (Δlog P) that is shifted by about 0.02 days with regard to the Milky Way OoI population, and the sample includes stars with Δlog P > 0.06 days, a characteristic hardly seen in Milky Way GCs. The metal-rich RR0 Lyrae stars in the Galactic bulge sample have Δlog P values on the other side of the spectrum as those in the metal-rich GCs NGC 6388 and NGC 6441. We find that the V amplitude for a given period is a function of Δlog P, and not of metal abundance, similar to the result found by Clement and Shelton for RR Lyrae stars in Milky Way GCs. A comparative study of the bulge field stars with similar metallicities but different Oosterhoff types is carried out. Bulge field RR0 Lyrae variables with Δlog P values similar to OoII clusters are about 0.2 mag brighter than RR0 Lyrae variables with Δlog P similar to OoI clusters. Reliance upon a single M RR V -[Fe/H] relation may be inappropriate when considering populations with different Δlog P.