[1] Univ Edinburgh, Royal Observ, Inst Astron, Edinburgh EH9 3HJ, Midlothian, Scotland
[2] Univ London Imperial Coll Sci Technol & Med, Blackett Lab, London SW7 2BZ, England
[3] Univ Cambridge, Inst Astron, Cambridge CB3 0HA, England
[4] Univ Durham, Dept Phys, Durham DH1 3LE, England
[5] Univ Oxford, Dept Astrophys, Oxford OX1 3RH, England
[6] Univ Hawaii, Inst Astron, Honolulu, HI 96822 USA
[7] Chinese Acad Sci, Beijing Astron Observ, Beijing 100080, Peoples R China
[8] Scottish Crop Res Inst, Dundee DD2 5DA, Scotland