1. Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, D-21029 Hamburg, Germany
2. Department of Physics and Astronomy, University of Bologna, Via Gobetti 93/2, I-40129 Bologna, Italy
3. Istituto di Radioastronomia, INAF, Via Gobetti 101, I-40129 Bologna, Italy
4. Department of Astrophysics/IMAPP, Radboud University, Postbus 9010, NL-6500 GL Nijmegen, the Netherlands