The Orbital Geometry of the Planet-hosting Binary Gamma Cep ABb

𝛾 Cep is a planet-hosting binary system consisting of an evolved intermediate-mass star (1.3 M☉) with a moderately wide stellar companion (𝛾 Cep B) at 20 AU and a planetary companion (𝛾 Cep b) orbiting the host star at 2 AU. The three-dimensional orbit of the stellar companion provides important constraints on the formation, migration, and stability of 𝛾 Cep b. We acquired new precise radial velocities from the Tull Coudé spectrograph on the Harlan J. Smith Telescope at McDonald Observatory which extends the RV baseline from 1987 to 2021. We also obtained adaptive optics images of 𝛾 Cep AB from Keck/NIRC2 in 2016, 2017, and 2019. The system additionally shows a strong astrometric acceleration between Hipparcos and Gaia EDR3. We used the python package orvara — Orbits from Radial Velocity, Absolute, and/or Relative Astrometry — to fit the three-body system. From the joint simultaneous fit, we obtained dynamical masses for 𝛾 Cep A and B and a precise inclination of the binary orbital plane. Using previous constraints on the planet’s orbital inclination, we find a strong misalignment between the planet and stellar orbital planes. This suggests that this system is likely undergoing Kozai-Lidov oscillations, in which each orbit is evolving together through exchanges of eccentricity and inclination.