The Eclipsing System Ep Andromedae and Its Circumbinary Companions

Abstract

We present new long-term CCD photometry for EP And acquired during the period 2007 to 2012. The light curves display total eclipses at primary minima and seasonto-season light variability. Our synthesis for all available light curves indicates that the eclipsing pair is a W-type overcontact binary with parameters of q=2.578, i=83 • .3, ∆T =27 K, f =28 %, and l 3 =2∼3 %. The asymmetric light curves in 2007 were satisfactorily modeled by a cool spot on either of the eclipsing components from a magnetic dynamo. Including our 95 timing measurements, a total of 414 times of minimum light spanning about 82 yr were used for a period study. A detailed analysis of the eclipse timing diagram revealed that the orbital period of EP And has varied as a combination of an upward-opening parabola and two periodic variations, with cycle lengths of P 3 =44.6 yr and P 4 =1.834 yr and semi-amplitudes of K 3 =0.0100 d and K 4 =0.0039 d, respectively. The observed period increase at a fractional rate of +1.39×10 −10 is in excellent agreement with that calculated from the W-D code and can be plausibly explained by some combination of mass transfer from the primary to the secondary star and angular momentum loss due to magnetic braking. The most reasonable explanation for both cycles is a pair of light-travel-time effects driven by the possible existence of a third and fourth component with projected masses of M 3 =0.25 M ⊙ and M 4 =0.90 M ⊙ . The more massive companion could be revealed using high-resolution spectroscopic data extending over the course of a few years and could also be a binary itself. It is possible that the circumbinary objects may have played an important role in the formation and evolution of the eclipsing pair, which would cause it to have a short initial orbital period and thus evolve into an overcontact configuration by angular momentum loss. Subject headings: binaries: close -binaries: eclipsing -stars: individual (EP Andromedae) -stars: spots 3. LIGHT-CURVE SYNTHESIS AND ABSOLUTE DIMENSIONS As shown in Figure 1, our observations display a typical light curve of an overcontact system and a flat bottom at primary minimum, indicating that the smaller primary star is totally occulted by the secondary. This would mean that EP And belongs to the W-type of W UMa stars. On the contrary, Pribulla et al. (2001) and Manzoori (2012) analyzed their BV light curves and both the AAVSO and WASP datasets, respectively, and classified the binary system as a member of the A-type category with the mass ratios of 0.34 and 0.395. The light curves of 2008 present equal light levels at the quadratures (Max I and Max II) within about 0.002 mag, while those of 2007 show the O'Connell effect with Max I brighter than Max II by about 0.014, 0.011, and 0.009 mag for the B, V , and R bandpasses, respectively. The effect is usually interpreted as spot activity on the component stars and the seasonal light variations most likely arise from the variability of the spots with time presumably produced by a magnetic dynamo.

The HU Aqr planetary system hypothesis revisited

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The active W UMa type binary star V781 Tau revisited

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The first light curve modeling and orbital period change investigation of nine contact binaries around the short period cut-off

“…The determined parameters for each system are displayed in Table 4, and the corresponding synthetic light curves, solid black lines, are plotted in Figure 3. We found that some of the targets manifest asymmetric light curves, in these cases spot model was applied to fit their light curves (e.g., Qian et al 2007;Lee et al 2013;Zhou et al 2016a). In addition, the search of a third light (l 3 ) was carried out for every system.…”