1. Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85740 Garching bei München, Germany
2. Centro de Astrofísica da Universidade do Porto, Rua das Estrelas - 4150-762 Porto, Portugal
3. Institut d'Astrophysique du CNRS, 98 bis Boulevard Arago, F-75014 Paris, France
4. Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA
5. Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
6. Apache Point Observatory PO Box 59, Sunspot, NM 88349-0059, USA