1. Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, D-21029 Hamburg, Germany
2. Dipartimento di Fisica e Astronomia, Universitá di Bologna, Via Gobetti 93/2, I-40121 Bologna, Italy
3. Istituto di Radio Astronomia, INAF, Via Gobetti 101, I-40121 Bologna, Italy