1. Special Astrophysical Observatory of the Russian Academy of Sciences, Nizhnij Arkhyz, Karachaevo-Cherkesia 369167, Russia
2. Astronomical Observatory, Volgina 7, 11160 Belgrade 74, Serbia
3. Isaac Newton Institute of Chile, Yugoslavia Branch, Volgina 7, Belgrade, Serbia
4. Department of Astronomy, Faculty of Mathematics, University of Belgrade, Studentski trg 16, 11000 Belgrade, Serbia
5. Instituto Nacional de Astrofísica, Óptica y Electrónica, Apartado Postal 51-216, 72000 Puebla, Puebla, Mexico
6. Sternberg Astronomical Institute of Lomonosov Moscow State University, 119992, Moscow, Russia
7. Max-Planck-Institut für Radioastronomie, Auf dem Hugel 69, D-53121 Bonn, Germany