1. Université Paris VII and Service d'Astrophysique, Commissariat à l'Energie Atomique CE-Saclay, 91191 Gif sur Yvette, France.
2. Astronomical Institute Anton Pannekoek, University of Amsterdam, and Center for High Energy Astrophysics, Kruislaan 403, 1098 SJ Amsterdam, Netherlands.
3. Australia Telescope National Facility, Commonwealth Scientific and Industrial Research Organization (CSIRO), Post Office Box 76, Epping NSW 1710, Australia.
4. Center for Astrophysics and Space Sciences, University of California at San Diego, MS 0424, La Jolla, CA 92093, USA.
5. Astronomical Institute, Utrecht University, Postbus 80000, 3508 TA Utrecht, Netherlands.
6. Center for Space Research, Massachusetts Institute of Technology, NE80-6055, 77 Massachusetts Avenue, Cambridge, MA 02139–4307, USA.
7. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA.