[1] Univ Leicester, Sch Phys & Astron, Astrophys Div, Leicester LE1 7RH, England
[2] Univ Amsterdam, Astron Inst Anton Pannekoek, Sci Pk 904, NL-1098 XH Amsterdam, Netherlands
[3] Leiden Univ, Leiden Observ, Niels Bohrweg 2, NL-2333 CA Leiden, Netherlands
[4] CNRS & Sorbonne Univ, Inst Astrophys Paris, UMR 7095, 98bis Bd Arago, F-75014 Paris, France
[5] Polish Acad Sci, Nicolaus Copernicus Astron Ctr, Ul Bartycka 18, PL-00716 Warsaw, Poland
[6] Univ Southampton, Dept Phys & Astron, Southampton SO17 1BJ, England