An Oosterhoff Analysis of the Galactic Bulge Field RR Lyrae stars: Implications On Their Absolute Magnitudes

The period‐V‐amplitude plane for fundamental mode pulsator RR Lyrae stars in the MACHO Bulge fields is examined. Only stars with “normal” light curves are used in the analysis. Although the RR Lyraes in Bulge globular clusters divide into two distinct groups based on the average periods [12], no gap between Oosterhoff I and II stars in the Bulge field star sample is seen. The main Δ log P distribution of the Bulge RR0 Lyrae field star population is shifted by about 0.02 days with regard to the Milky Way Oosterhoff I population. Stars with Δ log P>0.06 days are seen in the sample, a rare property of RR Lyrae in Milky Way globular clusters. A comparative study of the Bulge field stars with similar metallicities but different Oosterhoff types is carried out, and it is shown that Bulge field RR0 Lyrae variables with OoII‐like Δ log P values are about 0.2 mag brighter than RR0 Lyrae variables with OoI‐like Δ log P value. Hence, an RR Lyrae $MVRR−[Fe/H]$ relation may be inaccurate when considering populations with different Δ log P.