Institut für Theoretische Physik, Universität Frankfurt, D-60054 Frankfurt; bartosch(at)th.physik.uni-frankfurt.de
Received 27 July 2000, revised 7 February 2001,
accepted 27 February 2001 by U. Eckern
Enrico Fermi Institute, The University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA; stefan(at)gr.uchicago.edu
Received 21 November 2000, revised 26 February 2001,
accepted 22 March 2001 by K. Fredenhagen