1. Institute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences , Tsarigradsko Shose 72, BG-1784 Sofia, Bulgaria
2. Department of Astronomy, Sofia University ‘St. Kliment Ohridski’ , James Bourchier 5, BG-1164 Sofia, Bulgaria
3. Astronomical Association Sofia , Tsar Asen 49, BG-1463 Sofia, Bulgaria