1. Astrophysics Group, School of Physical and Geographical Sciences, Keele University, Keele, Staffordshire, ST5 5BG, UK
2. Observatoire Astronomique de Strasbourg, Université de Strasbourg, CNRS, UMR 7550, 11 rue de l'Université, F-67000 Strasbourg, France
3. Aix Marseille Univ, CNRS, LAM, Laboratoire d'Astrophysique de Marseille, F-13388 Marseille, France
4. Nicholas Copernicus Astronomical Center, ul. Bartycka 18, PL-00-716 Warsaw, Poland
5. Center for Astrophysics and Space Sciences, University of California, San Diego, MC 0424, La Jolla, CA 92093-0424, USA
6. INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy