Orbital period analysis of eclipsing Z Cam-type Dwarf Nova EM Cygni: Evidence of magnetic braking and a third body

摘要

Combining with our newest CCD times of light minimum of EM Cygni, all 45available times of light minimum including 7 data with large scatters arecompiled and the updated O-C analysis is made. The bestfit for the O-C diagramof EM Cygni is a quadratic-plus-sinusoidal fit. The secular orbital perioddecrease rate -2.5(\pm 0.3)x10^{-11} s s^{-1} means that magnetic brakingeffect with a rate of mass loss via stellar wind, 2.3x10^{-10}Msunyr^{-1}, isneeded for explaining the observed orbital period decrease. Moreover, forexplaining the significant cyclical period change with a period of \sim17.74(\pm 0.01)yr shown in the O-C diagram, magnetic activity cycles and lighttravel-time effect are discussed in detail. The O-C diagram of EM Cygni cannottotally rule the possibility of multi-periodic modulation out due to the gapspresented after 25000 cycles. Based on the hypothesis of a K-type third star inliterature, light trave-time effect may be a more plausible explanation.However, the low orbital inclination of the third body (\sim 7.4 degree)suggests that the hypothetic K-type third star may be captured by EM Cygni. Butassuming the spectral contamination from a block of circumbinary materialinstead of a K-type third star, the third star may be a brown dwarf in case ofthe coplanar orbit with parent binary.