Orbital Period Analysis of Eclipsing Z Cam-Type Dwarf Nova EM Cygni: Evidence of Magnetic Braking and a Third Body

Combining with our newest CCD times of light minimum of EM Cygni, all 45 available times of light minimum, including 7 data with large scatters, were compiled and an updated O - C analysis was made. The best-fit for the O - C diagram of EM Cygni was a quadratic-plus-sinusoidal fit. The secular orbital period decrease rate (-2.5 +/- 0.3) x 10(-11) s s(-1) means that a magnetic braking effect in EM Cygni with a mass-loss rate caused by stellar wind, 2.3 x 10(-10) M(circle dot) yr(-1), is needed for explaining the observed orbital period decrease. Moreover, for explaining the significant cyclical period change with a period of similar to 17.74 +/- 0.01 yr, shown in the O - C diagram, magnetic activity cycles and a light travel-time effect were considered in detail. The O - C diagram of EM Cygni cannot totally rule out the possibility of multi-periodic modulation due to gaps present after 25000 cycles. Based on the hypothesis of a K-type third star in the literature, a light trave-time effect may be a more plausible explanation. However, the low orbital inclination of the third body (similar to 7.degrees 4) suggests that the hypothetic K-type third star may be captured by EM Cygni. But, by assuming the spectral contamination from a block of circumbinary material instead of a K-type third star, the third star may be a brown dwarf in the case of a coplanar orbit with a parent binary.