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   51219
   Distant clusters of galaxies are important cosmological probes. The
   rapid cosmological evolution of clusters provides strong constraints on
   cosmological models. Massive distant clusters can act as gravitational
   lenses and can be used as "gravitational telescopes". Also distant
   clusters are used to study the evolution of galaxy populations in the
   past in a defined environment. We therefore propose a study of the
   three distant clusters of galaxies, which were found to be very bright
   in X-rays in the ROSAT Sky Survey. The observations should provide
   information on evolution of the luminosity-temperature and luminosity-
   metallicity relation which is an important test for cosmological
   models, and formation and evolution model of galaxies.
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