1. Institute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences, 72 Tsarigradsko Shose, 1784 Sofia, Bulgaria
2. Departamento de Física (EPSJ), Universidad de Jaén, Campus Las Lagunillas, A3-420, 23071 Jaén, Spain
3. Space Research and Technology Institute, Bulgarian Academy of Sciences, ul. Akad. G. Bonchev blok 1, 1113 Sofia, Bulgaria
4. Office of the Vice Chancellor, Botswana International University of Science and Technology, Private Bag 16, Palapye, Botswana