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 are compiled and the updated O-C analysis is made. The bestfit for the O-C diagram of EM Cygni is a quadratic-plus-sinusoidal fit. The secular orbital period decrease rate -2.5(\pm 0.3)x10^{-11} s s^{-1} means that magnetic braking effect with a rate of mass loss via stellar wind, 2.3x10^{-10}Msunyr^{-1}, is needed for explaining the observed orbital period decrease. Moreover, for explaining the significant cyclical period change with a period of \sim 17.74(\pm 0.01)yr shown in the O-C diagram, magnetic activity cycles and light travel-time effect are discussed in detail. The O-C diagram of EM Cygni cannot totally rule the possibility of multi-periodic modulation out due to the gaps presented after 25000 cycles. Based on the hypothesis of a K-type third star in literature, 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. But 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 case of the coplanar orbit with parent binary.Comment: 11 pages, 2 figures, accepted for publication in PAS

Quiescent photometric modulations of two low-inclination cataclysmic variables KZGem and TWVir

The quiescent periodic photometric modulations of two low-inclination cataclysmic variables observed in Kepler K2 Campaigns 0 and 1, KZ Gem and TW Vir, are investigated. A phase-correcting method was successfully used to detect the orbital modulations of KZ Gem and TW Vir and improve their orbital periods. The light curve morphologies of both CVs were further analyzed by defining flux ratios and creating colormaps. KZ Gem shows ellipsoidal modulations with an orbital period of 0.22242(1) day, twice the period listed in the updated RK catalogue (Edition 7.24). With this newly determined period, KZ Gem is no longer a CV in the period gap, but a long-period CV. A part of the quiescent light curve of TW Vir that had the highest stability was used to deduce its improved orbital period of 0.182682(3) day. The flat patterns shown in the colormaps of the flux ratios for KZ Gem demonstrate the stability of their orbital modulations, while TW Vir show variable orbital modulations during the K2 datasets. In TW Vir, the single versus double-peaked nature of the quiescent orbital variations before and after superoutburst may be related to the effect of the superoutburst on the accretion disk.Comment: 10 pages, 12 figures, accepted by A&

Evidence of a brown dwarf in the eclipsing dwarf nova Z Chamaeleonis

We presented three new CCD observations of light minima of Z Chamaeleonis. All 187 available times of light minimum including 37 photographic data are compiled, and a new orbital period analysis is made by means of the standard O - C technique. The O - C diagram of Z Chamaeleonis presents a cyclical periodic change of 32.57 yr with a high significance level. We attempted to apply two plausible mechanisms (i.e., Applegate's mechanism and light travel-time effect) to explain the cyclical variations of orbital period shown in the O-C diagram. Although the previous works suggested that solar-type magnetic cycles in the red dwarf are the best explanation, the analysis of Applegate's mechanism in this paper presents a negative result. Accordingly, a light travel-time effect is proposed, and a brown dwarf as a tertiary component orbiting around dwarf nova Z Chamaeleonis is derived with a significance level of ≳81.6%, which may be a plausible explanation of the periodic variation in the systemic velocity of Z Chamaeleonis in superoutburst.Facultad de Ciencias Astronómicas y Geofísica

Evidence of a brown dwarf in the eclipsing dwarf nova Z Chamaeleonis

We presented three new CCD observations of light minima of Z Chamaeleonis. All 187 available times of light minimum including 37 photographic data are compiled, and a new orbital period analysis is made by means of the standard O - C technique. The O - C diagram of Z Chamaeleonis presents a cyclical periodic change of 32.57 yr with a high significance level. We attempted to apply two plausible mechanisms (i.e., Applegate's mechanism and light travel-time effect) to explain the cyclical variations of orbital period shown in the O-C diagram. Although the previous works suggested that solar-type magnetic cycles in the red dwarf are the best explanation, the analysis of Applegate's mechanism in this paper presents a negative result. Accordingly, a light travel-time effect is proposed, and a brown dwarf as a tertiary component orbiting around dwarf nova Z Chamaeleonis is derived with a significance level of ≳81.6%, which may be a plausible explanation of the periodic variation in the systemic velocity of Z Chamaeleonis in superoutburst.Facultad de Ciencias Astronómicas y Geofísica

CCD photometric study of the W UMa-type binary II CMa in the field of Berkeley 33

The CCD photometric data of the EW-type binary, II CMa, which is a contact star in the field of the middle-aged open cluster Berkeley 33, are presented. The complete R light curve was obtained. In the present paper, using the five CCD epochs of light minimum (three of them are calculated from Mazur et al. (1993)'s data and two from our new data), the orbital period P was revised to 0.22919704 days. The complete R light curve was analyzed by using the 2003 version of W-D (Wilson-Devinney) program. It is found that this is a contact system with a mass ratio $q=0.9$ and a contact factor $f=4.1$. The high mass ratio ($q=0.9$) and the low contact factor ($f=4.1$) indicate that the system just evolved into the marginal contact stage