[1] Univ Montreal, Dept Phys, Montreal, PQ H3C 3J7, Canada
[2] Univ Arizona, Steward Observ, Tucson, AZ 85721 USA
[3] US Naval Observ, Flagstaff, AZ 86002 USA
[4] Princeton Univ Observ, Princeton, NJ 08544 USA
[5] Univ Washington, Dept Astron, Seattle, WA 98195 USA
[6] York Univ, Dept Phys & Astron, N York, ON M3J 1P3, Canada