Double cyclic variations in orbital period of the eclipsing cataclysmic variable EX Dra

Abstract

EX Dra is a long-period eclipsing dwarf nova with $\sim 2-3$ mag amplitude outbursts. This star has been monitored photometrically from November, 2009 to March, 2016 and 29 new mid-eclipse times were obtained. By using new data together with the published data, the best fit to the $O-C$ curve indicate that the orbital period of EX Dra have an upward parabolic change while undergoing double-cyclic variations with the periods of 21.4 and 3.99 years, respectively. The upward parabolic change reveals a long-term increase at a rate of $\dot{P}=+7.46\times 10^{-11}s{s}^{-1}$. The evolutionary theory of cataclysmic variables (CVs) predicts that, as a CV evolves, the orbital period should be decreasing rather than increasing. Secular increase can be explained as the mass transfer between the secondary and primary or may be just an observed part of a longer cyclic change. Most plausible explanation for the double-cyclic variations is a pair of light travel-time effect via the presence of two companions. Their masses are determined to be $M_{A}sin{i}_A^{\prime }=29.3(\pm 0.6)M_{Jup}$ and $M_{B}sin{i}_B^{\prime }=50.8(\pm 0.2)M_{Jup}$. When the two companions are coplanar to the orbital plane of the central eclipsing pair, their masses would match to brown dwarfs.

arXiv comment8 pages, 5 figures

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