1. Department of Astronomy, Faculty of Mathematics, University of Belgrade, Studentski trg 16, 11000 Belgrade, Serbia
2. Key Laboratory of Colleges and Universities in Yunnan Province for High-energy Astrophysics, Department of Physics, Yunnan Normal University, Kunming 650500, China
3. Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, Norman, OK 73019, USA
4. Astronomical Observatory Belgrade, Volgina 7, PO Box 74 11060, Belgrade 11060, Serbia