1. CERN, CH-1211 Geneva 23, Switzerland
2. Wigner Research Centre for Physics, Hungarian Academy of Sciences, H-1525 Budapest 114, P.O. Box 49, Hungary
3. Károly Róbert College, H-3200 Gyöngyös, Mátrai út 36, Hungary
4. Eötvös University, Department of Atomic Physics, H-1117 Budapest, Pázmány Péter s., 1/A, Hungary