1. Research Center for Astronomy and Applied Mathematics, Academy of Athens , 4 Soranou Efessiou Str., Athens 11527 , Greece
2. Astrophysics Science Division, NASA/ Goddard Space Flight Center , Greenbelt, MD 20771 , USA
3. Aristotle University of Thessaloniki, Department of Physics, Section of Astrophysics, Astronomy and Theoretical Mechanics , Thessaloniki 54124 , Greece