1. Department of Physics, Virginia Tech, Blacksburg, VA 24061, USA
2. Department of Physics and Astronomy, Bloomberg Center, Johns Hopkins University, Baltimore, MD 21218, USA
3. Department of Physics, Western Michigan University, 1120 Everett Tower, Kalamazoo, MI 49008-5252, USA
4. Isaac Newton Group of Telescopes, Apartado 321, E-38700 Santa Cruz de La Palma, Spain