Figure: Artist’s impression of a binary star system.
KOI-554.01, KOI-1074.01 and KOI-3728.01 are three transiting planet candidates detected by NASA's Kepler space telescope. More careful analysis of the data show that these three transiting planet candidates are actually false positive and should instead be classified as secondary components of binary star systems. Modulations detected in the out-of-transit region of the light curves of the host stars of KOI-554.01, KOI-1074.01 and KOI-3728.01 indicate that these three transiting planet candidates too massive to be planets.
These modulations are comprised of three components. (1) Ellipsoidal variations are caused by tides raised on the host star by the gravitational pull of the companion object, causing the host star to stretch into an ellipsoidal shape. (2) The companion object reflects light from its host star and also gives off its own thermal emission. (3) The gravitational tugging on the host star by the companion object causes Doppler beaming whereby the host star brightens slightly with it is approaching the observer and dims slightly when it is receding away. These three effects are collectively called REB modulations and they modulate the light curve of the host star depending on the position of the companion object in its orbit around the host star.
The amplitude of the REB modulations depend strongly on the mass of the companion object and its distance from it host star. Measuring the REB modulations can allow the mass and orbital distance of the companion object to be measured. A massive object in a close-in orbit around its host star can produce strong REB modulations. The REB modulations seen in the out-of-transit light curves show that KOI-554.01, KOI-1074.01 and KOI-3728.01 are low-mass stars orbiting much more massive hosts in binary star systems.
Figure 2: Mass-radius relationship for the three detected eclipsing binaries in the Kepler sample, namely KOI-554 (blue), KOI-1074 (green), and KOI-3728 (red). Primary stars are represented by squares, while the close companions are represented by circles. Other low-mass eclipsing binaries are also plotted with black plus symbols. Massive planets from the Exoplanet Catalogue are plotted as black crosses. Lillo-Box et al. (2016)
KOI-554.01 is estimated to have ~88 times the mass and ~0.77 times the radius of Jupiter, and its equilibrium temperature is estimated to be less than 2500 K. Its orbital period around its host star is 3.66 days. KOI-554.01 is most likely a low-mass red dwarf star, although it could also be a brown dwarf since its mass is near the stellar-substellar boundary. Its host star has an effective temperature of about 6108 K, and has ~1.13 times the mass and ~1.08 times the radius of the Sun.
KOI-1074.01 is a low-mass red dwarf star estimated to have ~155 times the mass and ~1.38 times the radius of Jupiter, and its equilibrium temperature is estimated to be less than 1500 K. Its orbital period around its host star is 3.77 days. Its host star has an effective temperature of about 6302 K, and has ~1.09 times the mass and ~1.30 times the radius of the Sun.
KOI-3728.01 is estimated to have ~83 times the mass and ~2.38 times the radius of Jupiter, and its equilibrium temperature is estimated to be less than 2100 K. Its orbital period around its host star is 5.55 days. KOI-3728.01 is most likely a low-mass red dwarf star, although it could also be a brown dwarf since its mass is near the stellar-substellar boundary. Its host star has an effective temperature of about 7360 K, and has ~2.10 times the mass and ~4.36 times the radius of the Sun. KOI-3728.01 also appears to be somewhat inflated in size.
Figure 3: For KOI-554.01. The top panel shows a close up view of the out-of-transit time interval where the light curve modulations are detectable. The middle panel shows the complex light curve including the eclipse and an inset showing each contribution to the out-of-transit modulations. Blue line - ellipsoidal variation; purple line - Doppler beaming; green line - reflectance. The bottom panel show the residuals of the fit. Lillo-Box et al. (2016)
Figure 4: Same as Figure 3, but for KOI-1074.01. Lillo-Box et al. (2016)
Figure 5: Same as Figure 4, but for KOI-3728.01. Lillo-Box et al. (2016)
Lillo-Box et al. (2016), "Search for light curve modulations among Kepler candidates. Three very low-mass transiting companions", arXiv:1606.02398 [astro-ph.EP]