1. Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, China
2. Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming 650216, China
3. University of Chinese Academy of Sciences, Beijing 100049, China
4. Hamburger Sternwarte, Universität Hamburg, Hamburg 21029, Germany
5. School of Physics and Astronomy, University of St Andrews, Fife KY16 9SS, UK
6. Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK