| Since RS CVn-type binary was defined by Hall in 1976, the solar-type activity(" chromospheric activity" or "magnetic activity") of contact binaries interests many astronomic researchers. Compared with other single stars, solar-type binaries,especially RS CVn-type binaries and BY Dra-type binaries, possess many extreme physical conditions: fast rotation velocity, synchronous rotation, transfer(or loss) of matter and energy, common envelope, and strong magnetic fields. Under the extreme physical conditions, solar-like binaries show all features of solar activity, but with great intensity: chromospheric emission lines (such as Hα,Hβ,CaⅡH & K,MgⅡh & k), X-ray emission, radio emission, spots, flare, anomalies in the continuum energy distribution (ultraviolet and infrared excess), mass loss in a wind, magnetic activity, and quasi-periodic (or irregular) orbital period variations. Observations show Hα,Hβ,CaⅡH & K,MgⅡh & k emission are strong and variational. WD code, Doppler imaging, and line depth ratio all show 2%-30% (maybe as much as 60%) of the surface of the components is coveredby spots (or the low temperature region). Quasi-periodic/irregular orbital period variations (with typical relative amplitude of△P/P - 10-7 - 10-5) can be only explained by magnetic activity. In all, there are strong magnetic fields in close binaries, especially chromospherically active binaries, and strong magnetic field is the origin of all solar-type activities.This paper has investigated magnetic activity of close binaries from theory and observation, and obtained the results as follow:1. In order to explain quasi-periodic/irregular variations of orbital period, we proposed a mechanism involving magnetic connection between two componentsof close binary. In the model, there is weak magnetic force between two component stars as result of small separation. As the magnetic fields are varying, the magnetic force and the orbital period also show variations. The model explainssuccessfully the quasi-periodic/irregular orbital period variations in some close binaries. What's more, the model can explain two variational components of orbital period seen in most contact binaries.2. The orbital periods of two early-type, massive contact binaries (V382 Cyg and TU Mus) were analyzed. It is found that the orbital periods increase at a ratios of +4.4×10-7days/year and +4.0×10-7days/year, respectively. Furthermore,the orbital periods also undergo cyclical oscillations with a period of 47.7 and 47.73 years, and an amplitude of 0.0148 and 0.0100 days. The two variational components can be explained by the mass transfer between two component stars and light-time effect arising from a tertiary star, or by magnetic activities, including the gravitational quadrupole model and magnetic connection model, can also explain the two variational components. According magnetic connection model, the required magnetic strength are 1.1T and 1.4 T, respectively.3. the light curves of two late-type contact binaries, EQ Tau and AD Cnc, obtained at different years cannot overlap, and show variational O'Connell effect. The light curves of EQ Tau show positive O'Connell effect in 2000, but negative O'Connell effect in 2001 and 2004. We propose that the variations of the light curves arise from the evolution of dark spots at hight latitude of the primary star. The light curves of AD Cnc is symmetrical in 1978, but show positive O'Connell effect in 2000 and negative O'Connell effect in 2002. We propose that the variations of the light curves arise from the evolution of dark spots at low latitude of the primary star. Moreover, the orbital periods of the two contact binaries show cyclical oscillations, which may arise from magnetic activities just as the dark spots do.4. the light curves of late-type contact binary, UX Eri, which are obtained in 1964 and 2003, can overlap each other near two maxima, but show a deviation of as much as 0.13 mag near second minimum. The variations of the light curves and orbital period may both arise from magnetic activities.5. On the basis of 71 contact binaries, which have one or more light curve(s), we find: (1) contact binaries evolve to low mass ratio; (2) late-type contact binaries are more magnetically active.
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