<P> RR Lyrae stars were formerly called "cluster variables" because of their strong (but not exclusive) association with globular clusters; conversely, over 80% of all variables known in globular clusters are RR Lyraes . RR Lyrae stars are found at all galactic latitudes, as opposed to classical Cepheids, which are strongly associated with the galactic plane . </P> <P> Several times as many RR Lyraes are known as all Cepheids combined; in the 1980s, about 1900 were known in globular clusters . Some estimates have about 85000 in the Milky Way . </P> <P> Though binary star systems are common for typical stars, RR Lyrae are very rarely observed in pairs . </P> <P> RR Lyrae stars pulse in a manner similar to Cepheid variables, but the nature and histories of these stars is thought to be rather different . Like all variables on the Cepheid instability strip, pulsations are caused by the κ - mechanism, when the opacity of ionised helium varies with its temperature . </P>

Rr lyrae stars share some common properties. all have about the same