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    PR # 31231                                                                   
     Rich clusters of galaxies exist at redshifts up to 1, but their typic       
     al X-ray luminosity drops significantly already at moderate redshifts       
      of 0.3-0.4. Observation of 2 clusters at z=0.3, A1758 and A1576, rep       
     resenting the extreme high end of the cluster X-ray luminosity functi       
     on, is proposed as part of a study of a well-defined sample of lumino       
     us distant X-ray selected clusters. The study is aimed at understandi       
     ng the nature of the negative evolution of the cluster X-ray luminosi       
     ty. Asca observations may test several interesting scenarios for clus       
     ter evolution.                                                              
                                                                                 
                                                                                 
                                                                                 
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