1. Department of Astronomy, University of Michigan, 1085 S. University Ave., Ann Arbor, MI 48109, USA
2. Department of Physics & Astronomy, Johns Hopkins University, Bloomberg Center, 3400 N. Charles St, Baltimore, MD 21218, USA
3. Max Planck Institut fur Astrophysik, Karl-Schwarzschild-Straße 1, D-85748 Garching bei München, Germany