Due to the lack of an accretion disk in a polar (magnetic cataclysmic variable, MCV), the material transferred from the secondary is directly accreted onto the white dwarf, forming an accretion stream and a hot spot on the white-dwarf component. During the eclipses, different light components can be isolated. Therefore, the monitoring of eclipsing polars could provide valuable information on several modern astrophysical problems, e.g., CVs as planetary hosting stars, mass transfer and mass accretion in CVs, and the magnetic activity of the most rapidly rotating cool dwarfs. In the past five years, we have monitored about 10 eclipsing polars (e.g., DP Leo and HU Aqr) using several 2-m class telescopes and about 100 eclipse profiles were obtained. In this paper, we will introduce the progress of our research group at YNOs. The first direct evidence of variable mass transfer in a CV is obtained and we show that it is the dark-spot activity that causes the mass transfer in CVs. Magnetic activity cycles of the cool secondary were detected and we show that the variable mass transfer is not caused by magnetic activity cycles. These results will shed light on the structure and evolution of close binary stars (e.g., CVs and Algols).

ABSTRACT
1. INTRODUCTION
2. POLARS AS A PLANETARY HOSTING STAR
3. CHARACTER OF MASS TRANSFER AND MASS AC-CRETION
4. MAGNETIC ACTIVITY OF THE RAPIDLY ROTAT-ING DWARFS
5. DISCUSSIONS AND CONCLUSIONS
REFERENCES

• S. -B. Qian(Yunnan Observatories, Chinese Academy of Sciences (CAS)/Key laboratory of the structure and evolution celestial bodies, Chinese Academy of Sciences/University of the Chinese Academy of Sciences)
• Z. -T. Han(Yunnan Observatories, Chinese Academy of Sciences (CAS)/Key laboratory of the structure and evolution celestial bodies, Chinese Academy of Sciences/University of the Chinese Academy of Sciences)
• L. -Y. Zhu(Yunnan Observatories, Chinese Academy of Sciences (CAS)/Key laboratory of the structure and evolution celestial bodies, Chinese Academy of Sciences/University of the Chinese Academy of Sciences)
• W. -P. Liao(Yunnan Observatories, Chinese Academy of Sciences (CAS)/Key laboratory of the structure and evolution celestial bodies, Chinese Academy of Sciences)
• E. Fernandez Lajus(Facultad de Ciencias Astronomicasy Geofsicas, Universidad Nacional de La Plata/Instituto de Astrosica de La Plata (CCT La plata - CONICET/UNLP))
• M. Zejda(Department of Theoretical Physics and Astrophysics, Masaryk University)
• L. Liu(Yunnan Observatories, Chinese Academy of Sciences (CAS)/Key laboratory of the structure and evolution celestial bodies, Chinese Academy of Sciences/University of the Chinese Academy of Sciences)
• B. Soonthornthum(National Astronomical Research Insititude of Thailand)
• X. Zhou(Yunnan Observatories, Chinese Academy of Sciences (CAS)/Key laboratory of the structure and evolution celestial bodies, Chinese Academy of Sciences/University of the Chinese Academy of Sciences)