1. Dipartimento di Fisica e Astronomia ‘G. Galilei’, University of Padova, Vicolo dell’Osservatorio 3, I–35122 Padova, Italy
2. INFN, Sezione di Padova, Via Marzolo 8, I–35131 Padova, Italy
3. INAF, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, I–35122 Padova, Italy
4. Institut für Astro- und Teilchenphysik, Universität Innsbruck, Technikerstrasse 25/8, A-6020, Innsbruck, Austria
5. Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA
6. Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
7. SISSA, via Bonomea 265, I-34136 Trieste, Italy