Department of Physics and Astronomy, Shumen University, 9700 Shumen, Bulgaria;
Department of Physics and Astronomy, Shumen University, 9700 Shumen, Bulgaria;
Department of Physics and Astronomy, Shumen University, 9700 Shumen, Bulgaria;
Medical University, Varna, 84 Tcar Osvoboditel str., Bulgaria;
Institute of Astronomy and NAO, Bulgarian Academy of Sciences, 72 Tsarigradsko Shose Blvd., 1784 Sofia, Bulgaria;