1. INAF-Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Piero Gobetti, 93/3, Bologna 40129, Italy
2. Universitá di Bologna, Dipartimento di Fisica e Astronomia, Via Piero Gobetti 93/2, I-40129, Bologna, Italy
3. Department of Physics, University of Bath, Claverton Down, Bath, BA2 7AY, UK
4. Observatories of the Carnegie Institution of Washington, 813 Santa Barbara St., Pasadena, CA 91101, USA
5. Department of Astronomy and Astrophysics, University of Chicago, 5640 S Ellis Ave, Chicago, IL 60637, USA
6. Department of Astronomy and Astrophysics, The Pennsylvania State University, 403 Davey Lab, University Park, PA 16802, USA