1. Department of Physics and Astronomy ‘Augusto Righi’, University of Bologna, via Gobetti 93/2, I-40129 Bologna, Italy
2. INAF-Astrophysics and Space Science Observatory of Bologna, via Gobetti 93/3, I-40129 Bologna, Italy
3. INAF-Osservatorio Astronomico di Brera, via Brera 28, 20121, I-Milano, Italy