The speed of light in confined QED vacuum: faster or slower than c?

We consider the propagation of light in the QED vacuum between an unusual p air of parallel plates, namely: a perfectly conducting one (epsilon --> inf inity) and an infinitely permeable one (mu --> infinity). For weak fields a nd in the soft photon approximation we show that the speed of light for pro pagation normal to the plates is smaller than its value in unbounded space (in contrast to the original Scharnhorst effect [K. Scharnhorst, Phys. Lett . B 236 (1990) 354, G. Barton, Phys, Lett. B 237 (1990) 559, G. Barton, K. Scharnhorst, J. Phys. A 26 (1993) 2037]). (C) 1999 Published by Elsevier Sc ience B.V. All rights reserved.