1. Astronomy and Physics Directorate, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109;
2. Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138;
3. Steward Observatory, University of Arizona, Tucson, Arizona 85721;
4. Astrophysics Branch, NASA/Ames Research Center, Moffett Field, California 94035;
5. Department of Physics and Astronomy, University of Rochester, Rochester, New York 14627;