[1] Univ Carthage, Grp Rech Phys Atom & Astrophys, Inst Natl Sci Appl & Technol, Ctr Urbain Nord, Tunis 1080, Tunisia
[2] UPMC, CNRS, Lab Etude Rayonnement & Mat Astrophys, Observ Paris,UMR 8112, F-92195 Meudon, France
[3] Astron Observ, Belgrade 11060 38, Serbia
[4] Isaac Newton Inst Chile, Yugoslavia Branch, Belgrade 11060, Serbia