1. Yunnan Observatories, China Academy of Sciences , Kunming 650216, China
2. Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences , Kunming 650011, China
3. Sternberg Astronomical Institute, M.V. Lomonosov Moscow State University , Leninskie Gory, Moscow 119991, Russia
4. Institute of Astronomy, KU Leuven , Celestijnenlaan 200D bus 2401, B-3001 Leuven, Belgium
5. Department of Astronomy, The Ohio State University , Columbus, OH 43210, USA