Tests of the Standard Model and Constraints on                                 
New Physics from Measurements of Fermion-pair                                  
Production at 189 GeV at LEP                                                   
  
        Cross-sections and angular distributions for hadronic and lepton pair  
        final states in e+e- collisions at centre-of-mass energy near          
        189 GeV, measured with the OPAL detector at LEP, are                   
        presented and compared with the predictions of the Standard Model.     
        The results are used to measure the energy dependence of the           
        electromagnetic coupling constant and to place limits on new physics   
        as described by four-fermion contact interactions or by the exchange   
        of a new heavy particle such as a sneutrino in supersymmetric theories 
        with R-parity violation.  A search for the indirect effects of the     
        gravitational interaction in extra dimensions on the mu-pair and       
        tau-pair final states is also presented.                               
  
 Parallel session :5                   
 Plenary  session :6a