1. INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy
2. Gran Sasso Science Institute, Viale F. Crispi 7, I-67100 L’Aquila, Italy
3. INFN/Laboratori Nazionali del Gran Sasso, Via G. Acitelli 22, I-67100 Assergi (AQ), Italy
4. Astronomical Observatory Volgina 7, 11060 Belgrade, Republic of Serbia
5. Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA