1. Dipartimento di Fisica e Astronomia, Università di Bologna, Via Gobetti 92/3, I-40129, Bologna, Italy
2. Institute of Radioastronomy – INAF, Via Gobetti 101, I-40129 Bologna, Italy
3. Hamburger Sternwarte, Gojenbergsweg 112, D-21029 Hamburg, Germany
4. Thüringer Landessternwarte, Sternwarte 5, D-07778 Tautenburg, Germany