1. Laboratory for Astrophysics, Leiden University, PO Box 9513, NL-2300, RA Leiden, The Netherlands
2. Leiden Observatory, Leiden University, 2300 RA Leiden, The Netherlands
3. Anton Pannekoek Institute, University of Amsterdam, Science Park 904, 1098XH Amsterdam, The Netherlands
4. Astronomy Department, University of Maryland, College Park, MD 20742, USA