Institution: | 1. Astronomical Observatory, Volgina 7, 11060 Belgrade 38, Serbia;2. Laboratoire d’Étude du Rayonnement et de la Matière en Astrophysique, Observatoire de Paris, UMR CNRS 8112, UPMC, 5 Place Jules Janssen, 92195 Meudon Cedex, France;3. Department of Applied Physics, Technical University-Sofia, 1000 Sofia, Bulgaria;4. Faculty of Mathematics, University of Belgrade, 11000 Belgrade, Serbia |