1. INAF–Osservatorio Astronomico di Brera, via Brera 28, I-20121 Milano, Italy
2. Green Bank Observatory, 155 Observatory Road, Green Bank, WV 24944, USA
3. Department of Physics and Astronomy, University of Pennsylvania, 209 South 33rd Street, Philadelphia, PA 19104, USA
4. IFPU - Institute for Fundamental Physics of the Universe, Via Beirut 2, I-34014 Trieste, Italy
5. INAF - Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, I-34143 Trieste, Italy
6. National Radio Astronomy Observatory, 520 Edgemont Rd, Charlottesville, VA 22903, USA
7. European Southern Observatory, Karl-Schwarzshild-Str. 2, D-85748 Garching b. München, Germany
8. Department of Astronomy, University of Virginia, P.O. Box 400325, Charlottesville, VA 22904, USA
9. Department of Physics, McGill University, 3600 University Street, Montreal, QC H3A 2T8, Canada