1. Dipartimento di Fisica e Astronomia, Via Gobetti 93/2, I-40129 Bologna, Italy
2. INAF-Astrophysics and Space Science Observatory Bologna, Via Gobetti 93/3, I-40129 Bologna, Italy
3. Department of Astronomy and Astrophysics, University of Toronto, 50 St George Str, Toronto ON M5S 3H4, Canada
4. Department of Astronomy, Indiana University, Swain West, 727 E 3rd Str, Bloomington, IN 47405, USA
5. European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei Munchen, Germany
6. Departamento de Ciencias Fisicas, Universidad Andres Bello, Republica 220, 837-0134 Santiago, Chile