1. Department of Physics and Astronomy, University of Alabama in Huntsville, Huntsville, AL 35899, USA
2. National Astronomical Observatory of Japan, 2-21-1, Osawa, Mitaka, Tokyo 181-8588, Japan
3. Dipartimento di Fisica G. Occhialini, Università degli Studi di Milano Bicocca, Piazza della Scienza 3, Milano I-20126, Italy
4. INAF-Osservatorio Astronomico di Brera, via Brera 28, Milano I-20121, Italy
5. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
6. Astronomical Institute of the Czech Academy of Sciences, Boční II 1401, Prague 141 00, Czech Republic
7. Max Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, Garching D-85741, Germany
8. Space Research Institute (IKI), Profsoyuznaya 84/32, Moscow 117997, Russia
9. Department of Astronomy and Astrophysics, The University of Chicago, Chicago, IL 60637, USA
10. Aix Marseille Université, CNRS, LAM (Laboratoire d’ Astro physique de Marseille) UMR 7326, Marseille F-13388, France
11. Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China