1. Department of Physics and Astronomy, Università di Padova, vicolo dell’Osservatorio 3, I-35122 Padova, Italy
2. Department of Astronomy, University of Wisconsin, 475 N. Charter Street, Madison, WI 53704, USA
3. INAF - Osservatorio di Padova, vicolo dell’ Osservatorio 5, I-35122 Padova, Italy
4. Department of Physics, University of the Free State, 9300 Bloemfontein, South Africa
5. Sternberg Astrophysical Institute, Lomonosov Moscow University, Universitetsky Ave., 13, Moscow 119992, Russia
6. Astronomical Institute of the Slovak Academy of Sciences, Tatranská Lomnica, 059 60, The Slovak Republic
7. Special Astrophysical Observatory, Russian Academy of Sciences, Nizhnij Arkhyz, Karachai-Cherkesia 369167, Russia
8. Kazan Federal University, Kremlevskaya 18, 420008 Kazan, Russia