Kepler-413b: A Slightly Misaligned, Neptune-size Transiting Circumbinary Planet

We report the discovery of a transiting, R_p = 4.347 ± 0.099R _⊕, circumbinary planet (CBP) orbiting the Kepler K+M eclipsing binary (EB) system KIC 12351927 (Kepler-413) every ~66 days on an eccentric orbit with a_p = 0.355 ± 0.002 AU, e_p = 0.118 ± 0.002. The two stars, with M_A = 0.820 ± 0.015 M _⊙, R_A = 0.776 ± 0.009 R _⊙ and M_B = 0.542 ± 0.008 M _⊙, R_B = 0.484 ± 0.024 R _⊙, respectively, revolve around each other every 10.11615 ± 0.00001 days on a nearly circular (e _EB = 0.037 ± 0.002) orbit. The orbital plane of the EB is slightly inclined to the line of sight (i _EB = 87.°33 ± 0.°06), while that of the planet is inclined by ~2.°5 to the binary plane at the reference epoch. Orbital precession with a period of ~11 yr causes the inclination of the latter to the sky plane to continuously change. As a result, the planet often fails to transit the primary star at inferior conjunction, causing stretches of hundreds of days with no transits (corresponding to multiple planetary orbital periods). We predict that the next transit will not occur until 2020. The orbital configuration of the system places the planet slightly closer to its host stars than the inner edge of the extended habitable zone. Additionally, the orbital configuration of the system is such that the CBP may experience Cassini State dynamics under the influence of the EB, in which the planet's obliquity precesses with a rate comparable to its orbital precession. Depending on the angular precession frequency of the CBP, it could potentially undergo obliquity fluctuations of dozens of degrees (and complex seasonal cycles) on precession timescales.