We would like to thank Jean-Michel Alimi for discussions, and Arno G. Weiß for constructive contributions and discussions. GK and TB were both supported by the "Sonderforschungsbereich 375 für Astro-Teilchenphysik der Deutschen Forschungsgemeinschaft". All authors acknowledge financial support from a DAAD exchange program.
© European Southern Observatory (ESO) 1997
Online publication: July 8, 1998