[1] Peking Univ, Kavli Inst Astron & Astrophys, Beijing 100871, Peoples R China
[2] Univ Cent Lancashire, Jeremiah Horrocks Inst, Preston PR1 2HE, Lancs, England
[3] Peking Univ, Sch Phys, Dept Astron, Beijing 100871, Peoples R China
[4] Natl Res Council Canada, Herzberg Inst Astrophys, Victoria, BC V9E 2E7, Canada
[5] Univ Victoria, Dept Phys & Astron, Victoria, BC V8P 5C2, Canada
[6] Max Planck Insitut Extraterrr Phys, Giessenbachstr, D-85748 Garching, Germany
[7] McMaster Univ, Dept Phys & Astron, Hamilton, ON L8S 4M1, Canada
[8] Univ Washington, Astron Dept, Seattle, WA 98195 USA
[9] Univ Malta, Dept Phys, MSD-2080 Msida, Malta
[10] Univ Malta, Inst Space Sci & Astron, MSD-2080 Msida, Malta