1. Harvard-Smithsonian Center for Astrophysics, 60 Garden St, Cambridge, MA 02138, USA
2. University of Texas at Austin, 110 Inner Campus Dr, Austin, TX 78705, USA
3. Department of Astronomy, University of California, Berkeley, CA 94720, USA
4. Max–Planck–Institut für Astrophysik, Karl–Schwarzschild–Str. 1, D-85741 Garching, Germany