1. Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, UK
2. School of Physics, Trinity College Dublin, Dublin 2, Ireland
3. Instituto de Astrofísica de Canarias, C/Vía Láctea s/n, E-38200 La Laguna, Spain
4. Departamento de Astrofísica, Universidad de La Laguna, Avenida Astrofísico Francisco Sánchez, s/n, E-38206 La Laguna, Spain
5. Astronomical Institute, School of Science, Tohoku University, Sendai 980-8578, Japan