1. Leiden Observatory, Leiden University, PO Box 9513, NL-2300 RA Leiden, the Netherlands
2. Netherlands Institute for Radio Astronomy (ASTRON), Postbus 2, NL-7990 AA Dwingeloo, the Netherlands
3. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
4. Department of Astronomy, University of Maryland, College Park, MD 20742, USA