[1] Univ Durham, Dept Phys, Durham DH1 3LE, England
[2] Univ Hertfordshire, Dept Phys Sci, Hatfield AL10 9AB, Herts, England
[3] Univ Kentucky, Dept Phys & Astron, Lexington, KY 40506 USA
[4] Univ Montreal, Observ Mont Megant, Montreal, PQ H3C 3J7, Canada
[5] Univ Montreal, Dept Phys, Montreal, PQ H3C 3J7, Canada
[6] Inst Astrofis Canarias, E-38200 San Cristobal la Laguna, Tenerife, Spain