1. Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
2. Department of Physics, University of Warwick, Coventry CV4 7AL, UK
3. INAF/Brera Astronomical Observatory, via Bianchi 46, I-23807, Merate (LC), Italy
4. Department of Physics and Astronomy, University of Sheffield, Hounsfield Rd, Sheffield S3 7RH, UK
5. Warsaw University Astronomical Observatory, Al. Ujazdowskie 4, PL-00-478 Warszawa, Poland
6. Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK