KIC 8560861 ( HD 183648 ) is a marginally eccentric ( e = 0.05 ) eclipsing binary with an orbital period of P _ { \mathrm { orb } } = 31.973 d , exhibiting mmag amplitude pulsations on time scales of a few days .
We present the results of the complex analysis of high and medium-resolution spectroscopic data and Kepler Q0 – Q16 long cadence photometry .
The iterative combination of spectral disentangling , atmospheric analysis , radial velocity and eclipse timing variation studies , separation of pulsational features of the light curve , and binary light curve analysis led to the accurate determination of the fundamental stellar parameters .
We found that the binary is composed of two main sequence stars with an age of 0.9 \pm 0.2 Gyr , having masses , radii and temperatures of M _ { 1 } = 1.93 \pm 0.12 M _ { \odot } , R _ { 1 } = 3.30 \pm 0.07 R _ { \odot } , T _ { \mathrm { eff 1 } } = 7650 \pm 100 K for the primary , and M _ { 2 } = 1.06 \pm 0.08 M _ { \odot } , R _ { 2 } = 1.11 \pm 0.03 R _ { \odot } , T _ { \mathrm { eff 2 } } = 6450 \pm 100 K for the secondary .
After subtracting the binary model , we found three independent frequencies , two of which are separated by twice the orbital frequency .
We also found an enigmatic half orbital period sinusoidal variation that we attribute to an anomalous ellipsoidal effect .
Both of these observations indicate that tidal effects are strongly influencing the luminosity variations of HD 183648 .
The analysis of the eclipse timing variations revealed both a parabolic trend , and apsidal motion with a period of P _ { \mathrm { apse } } ^ { \mathrm { obs } } = 10 400 \pm 3 000 y , which is three times faster than what is theoretically expected .
These findings might indicate the presence of a distant , unseen companion .