1. Department of Physics, University of Crete, GR-70013 Heraklion, Greece
2. Institute of Astrophysics, FORTH, GR-71110 Heraklion, Greece
3. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
4. Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, National Observatory of Athens, 15236 Penteli, Greece