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B.J. Pritzl, H.A. Smith (Michigan State University), M. Catelan (University of Virginia), A.V. Sweigart (NASA/GSFC)
NGC 6388 and NGC 6441 are metal-rich globular clusters which show the second parameter effect. Each cluster has a prominent blue extension to its horizontal branch (HB), not typically associated with metal-rich globular clusters (Rich et al. 1997). Moreover, this extension slopes upward in V with decreasing B-V color. Canonical models with a greater cluster age or enhanced mass loss along the red giant branch cannot account for this behavior (Sweigart & Catelan 1998). Sweigart and Catelan proposed several noncanonical models to explain this effect. We present new B,V light curves for the RR Lyrae stars in these clusters in order to test these new models and their implications for the second parameter effect. We have found that these variables have unusually long periods as compared to field stars of similar metallicity, suggesting unusually bright luminosities. Our results indicate that a universal relationship may not exist between the luminosity and the metallicity of RR Lyrae variables. Implications for the Oosterhoff phenomenon will also be briefly discussed.
We thank NSF and NASA, for partial support, of this research.