Photonic Events with Missing Energy in                                         
e+e- Collisions at sqrt(s) = 189 GeV                                           
  
Photonic events with missing energy have been observed in 189 GeV              
e+e- collisions using the OPAL detector at LEP. Results are presented          
for single-photon and acoplanar-photons event topologies.                      
Cross-section measurements are performed and the number of light               
neutrino species is measured. Upper limits on cross-section times              
branching ratios for the class of new physics models e+ e- -> XX or            
XY are derived for stable and invisible Y.  Macroscopic decay lengths          
are considered in XX production when Y is nearly massless. Constraints         
are placed on superlight gravitino pair production as well as                  
graviton-photon production in models with additional space dimensions.         
  
 Parallel session :11                  
 Plenary  session :7a