2022ApJ...924...66Y

We explore the fascinating eclipses and dynamics of the compact hierarchical triple-star system KOI-126 (KIC 5897826). This system is composed of a pair of M-dwarf stars (KOI-126 B and C) in a 1.74 day orbit that revolve around an F star (KOI-126 A) every 34 days. Complex eclipse shapes are created as the M stars transit the F star, due to two effects: (1) the duration of the eclipse is a significant fraction of the M-star orbital period, so the prograde or retrograde motion of the M stars in their orbit lead to unusually short or long duration eclipses; (2) due to 3-body dynamics, the M-star orbit precesses with an astonishingly quick timescale of 1.74 yr for the periastron (apsidal) precession, and 2.73 yr for the inclination and nodal angle precession. Using the full Kepler data set, supplemented with ground-based photometry, plus 29 radial velocity measurements that span 6 yr, our photodynamical modeling yields masses of M_A = 1.2713 ± 0.0047 M_☉ (0.37%), M_B = 0.23529 ± 0.00062 M_☉ (0.26%), and M_C = 0.20739 ± 0.00055 M_☉ (0.27%) and radii of R_A = 1.9984 ± 0.0027 R_☉ (0.14%), R_B = 0.25504 ± 0.00076 R_☉ (0.3%), and R_C = 0.23196 ± 0.00069 R_☉ (0.3%). We also estimate the apsidal motion constant of the M dwarfs, a parameter that characterizes the internal mass distribution. Although it is not particularly precise, we measure a mean apsidal motion constant, , of 0.046_–0.028^+0.046, which is approximately 2σ lower than the theoretical model prediction of 0.150. We explore possible causes for this discrepancy.