RR Lyrae stars in NGC 6388 and NGC 6441: A new Oosterhoff group?

NGC 6388 and NGC 6441 are anomalies among Galactic globular clusters in that they cannot be readily placed into either Oosterhoff group I or Oosterhoff group II despite their significant numbers of RR Lyrae variables. The mean pulsation periods, [P-ab], of their RRab variables, at 0.71 and 0.76 days, respectively, are even larger than for Oosterhoff II clusters. Moreover, Oosterhoff II clusters are very metal poor, whereas NGC 6388 and NGC 6441 are the most metal-rich globular clusters known to contain RR Lyrae stars. The location of the NGC 6388 and NGC 6441 RRab variables in the period-amplitude diagram implies that the RR Lyrae stars in those two clusters are brighter than expected for their metallicities. Our results therefore indicate that a universal relationship may not exist between the luminosity and the metallicity of RR Lyrae variables.