1. Laboratori Nazionali del Sud INFN, I-95123 Catania, Italy
2. Physics-Astronomy Dept., University of Catania, Italy
3. Dept. of Theoretical Physics, Bucharest Univ., and NIPNE-HH, Magurele, Bucharest, Romania
4. Inst. für Theoretische Physik, Universität Giessen, D-35312 Giessen, Germany
5. Dept. of Theoretical Physics, Aristotle Univ. of Thessaloniki, Gr-54124, Greece
6. Dept. für Physik, Universität München, D-85748 Garching, Germany
7. Smith College, Northampton, Mass., USA