1. International Centre for Theoretical Physics, Trieste, Italy
2. Institute of Physics, University of Zagreb, P.O. Box 304, 41001 Zagreb, Yugoslavia
3. Universität Duisburg, FB 10 Physik-Technologie, Postfach 10 16 29, D-4100 Duisburg, Federal Republic of Germany
4. TU Dresden, WB Theoretische Physik, Mommsenstr. 13, DDR-8027 Dresden, German Democratic Republic