2018A&A...615L..13G

Context. We present the transit and follow-up of a single transit event from Campaign 14 of K2, EPIC248847494b, which has a duration of 54 h and a 0.18% depth.
Aims. Using photometric tools and conducting radial velocity follow-up, we vet and characterise this very strong candidate.
Methods. Owing to the long, unknown period, standard follow-up methods needed to be adapted. The transit was fitted using Namaste, and the radial velocity slope was measured and compared to a grid of planet-like orbits with varying masses and periods. These used stellar parameters measured from spectra and the distance as measured by Gaia.
Results. Orbiting around a sub-giant star with a radius of 2.70±0.12R_☉, the planet has a radius of 1.11_–0.07^+0.07R_Jup and a period of 3650_–1130⁺¹²⁸⁰-days. The radial velocity measurements constrain the mass to be lower than 13M_Jup, which implies a planet-like object.
Conclusions. We have found a planet at 4.5AU from a single-transit event. After a full radial velocity follow-up campaign, if confirmed, it will be the longest-period transiting planet discovered.