1. Department of Astronomy, School of Physics, Peking University, Beijing 100871, P. R. China
2. Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, P. R. China
3. Centre for Space Research, Physics Department, North-West University, Mahikeng 2745, South Africa
4. Jeremiah Horrocks Institute, University of Central Lancashire, Preston PR1 2HE, UK
5. Astronomical Institute, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
6. Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, P-4150-762 Porto, Portugal
7. Department of Astronomy, Beijing Normal University, Beijing 100875, P. R. China
8. Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, PL-00-716 Warsaw, Poland