1. Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, D-21029 Hamburg, Germany
2. INAF-Istituto di Radioastronomia, via P. Gobetti, 101, I-40129 Bologna, Italy
3. Leiden Observatory, Leiden University, Oort Gebouw, PO Box 9513, NL-2300 RA Leiden, the Netherlands
4. International Centre for Radio Astronomy Research – Curtin University, GPO Box U1987, Perth, WA 6845, Australia
5. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
6. Dipartimento di Fisica e Astronomia, Universit di Bologna, via P. Gobetti 93/2, I-40129 Bologna, Italy
7. Netherlands Institute for Radio Astronomy (ASTRON), PO Box 2, NL-7990 AA Dwingeloo, the Netherlands