1. School of Physics and Astronomy, University of St Andrews, North Haugh, KY16 9SS, UK (SUPA)
2. Instituto de Astrofísica de Canarias (IAC), E-38205 La Laguna, Tenerife, Spain
3. Universidad de La Laguna (ULL), Departamento de Astrofísica, E-38206 La Laguna, Tenerife, Spain
4. University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
5. Department of Astrophysical Sciences, Peyton Hall, Princeton University, Princeton, NJ 08544, USA
6. INAF Osservatorio Astronomico di Brera, via Brera, 28, I-20159 Milano, Italy
7. Dipartimento di Fisica e Astronomia G. Galilei, Universita di Padova, vicolo dell Osservatorio 3, I-35122 Padova, Italy