RR Lyrae Period-Amplitude Diagrams: From Bailey to Today
  More than a century ago, Solon Bailey's pioneering investigations of the variable stars in globular clusters allowed the first period-amplitude diagrams to be constructed for their RR Lyrae stars. These diagrams differ from cluster to cluster, and there has been debate as to whether these differences are correlated mainly with [Fe/H] or with Oosterhoff type. It is clear now that a cluster's Oosterhoff type plays an important role in determining its period-amplitude relation, although the Oosterhoff dichotomy itself is correlated with metallicity. Not all clusters follow the usual patterns, however. The globular clusters NGC 6388 and NGC 6441 have period-amplitude diagrams similar to those of metal-poor Oosterhoff type II globular clusters, but they themselves are comparatively metal-rich. The period-amplitude diagrams of Oosterhoff-intermediate systems are discussed. 
Horace A. Smith, Marcio Catelan, Charles Kuehn
astro-ph.SR