1. Department of Astrophysics/IMAPP, Radboud University, P.O. Box 9010, NL-6500 GL Nijmegen, The Netherlands
2. Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, NL-1098 XH Amsterdam, The Netherlands
3. H. H. Wills Physics Laboratory, Tyndall Avenue, Bristol BS8 1TL, UK