[1] Univ Cambridge, Inst Astron, Cambridge CB3 0HA, England
[2] Dominion Astrophys Observ, Victoria, BC V8X 4M6, Canada
[3] Univ Toronto, Dept Astron, Toronto, ON, Canada
[4] Astron Spatiale Lab, F-13376 Marseille, France
[5] Anglo Australian Observ, Mt Stromlo & Siding Spring Observ, Coonabarabran, NSW 2357, Australia
[6] Observ Paris, Sect Meudon, DAEC, F-92195 Meudon, France
[7] Australian Natl Univ, Mt Stromlo & Siding Spring Observ, Weston, ACT 2611, Australia
[8] Univ Calif Berkeley, Dept Astron, Berkeley, CA 94720 USA