On The Orbital Period Changes Of RS CVn-Type Binaries CG Cyg And V471Tau

There are many kinds of close binaries, but RS CVn-type close binary has the uniquephysical properties. RS CVn-type close binary is a chromospherically active system withstrong magnetic activity. In some cases, RS CVn type close binary show many features ofsolar magnetic activity, such as: strong H and CaII H&K emission lines, star-spotactivity, flare, mass loss due to star wind etc. These activities affect directly or indirectlythe orbital period change of the system.In this paper, we have introduced the methods of researching the orbital periodchange of RS CVn-type binary and the mechanisms that cause the period change, andthen we have analyzed the orbital period changes of RS CVn-type binary CG Cyg andV471Tau in detail. Our main results are as follows:1. The orbital period change of the RS CVn type binary CG Cyg was analyzed. Firstof all, we used the ephemeris proposed by Milone&Ziebarth in1974to calculate theO Cvalue. We have used the least squares to fit the O Ccurve. We found that theorbital period of CG Cyg undergo change with two modulation periods of61.63yr,20.34yr and a long-term increase with a rate ofdP/dt9.77（2.17）109dyr^-1; then weused a new ephemeris to calculate the O Cvalue, and got a new O Ccurve. We usedsimilarly the least squares to fit the new O Ccurve, we got two modulation periods of18.9yr and21.6yr, and its long-term increase rate of period isdP/dt2.41（0.17）108dyr^-1. These two groups of results are clearly not consistent. Inorder to prove these results, we used the polynomial method to fit the O Ccurvesrespectively. The calculation results show that, there were two modulation periodsofP（mod1）127.8yrandP（mod）263.9yrby using the ephemeris that proposed by Milone&Ziebarth in1974to calculate O Ccurve, andP（mod1）129.2yrandP（mod）264.6yrbyusing the new ephemeris to calculate O Ccurve, meanwhile they were accompanied bya long-term increase of period with the rate ofdP/dt2.48108dyr^-1, and the qualitytransfer rate of the binary wasdM/dt1.2210^-8M11⊙yr. Based on these results, weanalyzed the reason of the variable orbital period of CG Cyg, the results were that thesystem does not exist the third or fourth bodies, the modulation periods may be caused bymagnetic activity of binary stars, and the long-term increase of period may be due tomaterial exchange between the primary component and the quality center of binary stars.2. The RS CVn type binary V471Tau orbital period was analyzed. Through theanalysis, we concluded that there was a modulation period of V471Tau was31.1yr, its corresponding amplitudes was PΔ=5^d.59（0d.64）10^-7. In conclusion, we knew that theperiod change of the binary systems might be caused by the magnetic activity in stead ofthe light time effect due to the third body.