Department of Physics and Astronomy University of Shumen 115 Universitetska Str. 9700 Shumen Bulgaria;
Department of Physics and Astronomy University of Shumen 115 Universitetska Str. 9700 Shumen Bulgaria;
Institute of Astronomy and National Astronomical Observatory Bulgarian Academy of Sciences 72 Tsarigradsko Shose Blvd. 1784 Sofia Bulgaria;
Institute of Astronomy and National Astronomical Observatory Bulgarian Academy of Sciences 72 Tsarigradsko Shose Blvd. 1784 Sofia Bulgaria;