1Institut für Theoretische Physik IV, Universität
Düsseldorf, D-40225 Düsseldorf
2Center for Theoretical Physics, 02-668 Warszawa, Poland
*E-mail: bausch(at)thphy.uni-duesseldorf.de
Received 9 November 1998, accepted 23 November 1998 by U. Eckern
Physikalisches Institut, Universität Karlsuhe, D-76128 Karlsruhe,
Germany
E-mail: bernd(at)piobelix.physik.uni-karlsruhe.de
Received 15 January 1999, accepted 29 January 1999 by U. Eckern