1. Max-Planck Institute for Astrophysics, Karl-Schwarzschild Str 1, D-85748 Garching, Germany
2. Kapteyn Astronomical Institute, University of Groningen, Postbus 800, NL-9700 AV Groningen, the Netherlands
3. Dipartimento di Fisica e Astronomia, Universita di Bologna, 6/2, Viale Berti Pichat, I-40127 Bologna, Italy
4. Research School of Astronomy and Astrophysics, The Australian National University, Canberra, ACT 2611, Australia