1. National University of Public Service, H-1441, Budapest, Hungary
2. I. Physikalisches Institut, Universität zu Köln, Zülpicher-Str. 77, D-50937 Cologne, Germany
3. Department of Physics and Astronomy of College of Charleston, Charleston, SC 29424, USA
4. Universität Hamburg, Fachbereich Mathematik, Bundesstr. 55, D-20146 Hamburg, Germany
5. Department of Physics of Complex Systems the Eötvös Loránd University, Pázmány Péter sétány 1./A, 1117 Budapest, Hungary
6. Department of Astronomy of the Eötvös Loránd University, Pázmány Péter sétány 1./A, 1117 Budapest, Hungary