[1] Queens Univ Belfast, Dept Phys & Astron, Astrophys & Planetary Sci Res Div, Belfast BT7 1NN, Antrim, North Ireland
[2] Isaac Newton Grp Telescope, Tenerife, Spain
[3] Open Univ, Dept Phys & Astron, Milton Keynes MK7 6AA, Bucks, England
[4] Univ St Andrews, Sch Phys & Astron, St Andrews KY16 9SS, Fife, Scotland
[5] Univ Leicester, Dept Phys & Astron, Leicester LE1 7RH, Leics, England
[6] Univ Keele, Sch Chem & Phys, Astrophys Grp, Keele ST5 5BG, Staffs, England
[7] Univ Cambridge, Inst Astron, Cambridge CB3 0HA, England
[8] Univ Warwick, Dept Phys, Coventry CV4 7AL, W Midlands, England