1. INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, I-40127 Bologna, Italy
2. Dipartimento di Fisica e Astronomia, Università di Bologna, viale Berti Pichat 6/2, I-40127 Bologna, Italy
3. INFN, Sezione di Bologna, viale Berti Pichat 6/2, I-40127 Bologna, Italia
4. Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
5. Institute of Astronomy and Astrophysics, Academia Sinica, PO Box 23-141, Taipei 10617, Taiwan
6. Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
7. Physics Department, Ben-Gurion University of the Negev, PO Box 653, Be'er-Sheva 84105, Israel