[1] Astron Observ, Volgina 7, Belgrade 11060, Serbia
[2] IHIS Techno Experts, Bezanijski Put 23, Belgrade 11080, Serbia
[3] Univ Paris 06, Sorbonne Univ, Observ Paris, LERMA,PSL Res Univ,CNRS, 5 Pl Jules Janssen, F-92195 Meudon, France
[4] Inst Isaac Newton, Yugoslavia Branch, Belgrade 11060, Serbia
[5] Tech Univ Sofia, Dept Appl Phys, Sofia 1000, Bulgaria
[6] Fac Math, Dept Astron, Studentski Trg 16, Belgrade 11000, Serbia