1. Nicolaus Copernicus Astronomical Center of the Polish Academy of Sciences, Bartycka 18, PL-00-716 Warszawa, Poland
2. Center for Astrophysics and Space Sciences, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0424, USA
3. Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse, D-85741 Garching, Germany