1. Dipartimento di Fisica e Astronomia, Università di Bologna, via P. Gobetti 93/2, I-40129 Bologna, Italy
2. INAF – IRA, via P. Gobetti 101, I-40129 Bologna, Italy
3. ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, NL-7990 AA Dwingeloo, the Netherlands
4. Leiden Observatory, Leiden University, PO Box 9513, NL-2300 RA Leiden, the Netherlands
5. Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, D-21029 Hamburg, Germany
6. Thüringer Landessternwarte, Sternwarte 5, D-07778 Tautenburg, Germany
7. Department of Physical Sciences, The Open University, Milton Keynes MK7 6AA, UK
8. Space Science and Technology Department, The Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0NL, UK