1. Yunnan Observatories, Chinese Academy of Sciences (CAS), Kunming 650216, P.R. China
2. Key Laboratory for the Structure and Evolution of Celestial Objects, CAS, Kunming 650216, P.R. China
3. Center for Astronomical Mega-Science, CAS, Beijing 100012, P.R. China
4. Argelander-Institut für Astronomie, Auf dem Hügel 71, D-53121 Bonn, Germany