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 tha t they cannot be readily placed into either Oosterhoff group I or Oosterhof f group II despite their significant numbers of RR Lyrae variables. The mea n 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 a re the most metal-rich globular clusters known to contain RR Lyrae stars. T he location of the NGC 6388 and NGC 6441 RRab variables in the period-ampli tude diagram implies that the RR Lyrae stars in those two clusters are brig hter 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.