1. University of Central Lancashire, Jeremiah Horrocks Institute, Preston, PR1 2HE, UK
2. Max Planck Institute für Kernphysik, Saupfercheckweg 1, D-69117 Heidelberg, Germany
3. The Astronomical Institute of the Romanian Academy, Str. Cutitul de Argint 5, Bucharest, Romania
4. University of Science and Technology of China, 230026 Hefei, Anhui, China