1. Department of Astronomy, Yale University, 52 Hillhouse Avenue, New Haven, CT 06511, USA
2. Department of Physics, Yale University, PO Box 208121, New Haven, CT 06520, USA
3. Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747 AD Groningen, USA
4. Department of Physics, University of Miami, Coral Gables, FL 33124, USA
5. INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via Gobetti 93/3, I-40129 Bologna, Italy