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   PR # 41230
   Distant clusters of galaxies are important cosmological probes. The
   rapid evolution of clusters as observed in the evolution of the X-ray
   luminosity function provides strong contraints on cosmological
   models. The most massive, X-ray luminous clusters are the most
   important objects in this respect. We therefore propose to observe
   four extremely X-ray luminous galaxy clusters detected in the ROSAT
   All Sky Survey. We like to determine their gas temperatures and iron
   abundances for comparison of these properties with those of nearby
   clusters. In connection with scheduled ROSAT HRI observations we like
   to determine the cluster masses and gas mass fractions. These objects
   will also be important targets for gravitational lensing and
   Sunyaev-Zel'dovich effect studies.
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