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   61194
   Distant clusters of galaxies are important cosmological probes. The
   cosmological evolution of clusters provides strong constraints on
   cosmological models. Also, distant clusters are used to study the
   evolution of galaxy populations in the past. The strong correlation of
   the X-ray luminosity with cluster mass helps to select the most massive
   clusters from X-ray surveys. We therefore propose a study of four
   luminous distant clusters of galaxies at $z\\sim0.3-0.4$, which were
   found in the ROSAT Sky Survey. The observations should provide the gas
   temperature and metallicity which can be compared to the corresponding
   properties of present-day clusters with similar X-ray luminosities.
   These objects will be added to our sample of distant clusters to study
   the cosmological evolution.
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