Kepler photometric time series for KIC 4918810

We report the detection of a single transit-like signal in the Kepler data of the slightly evolved F star KIC 4918810. The transit duration is ~45 hours, and while the orbital period (P~10 years) is not well constrained, it is one of the longest among companions known to transit. We calculate the size of the transiting object to be Rp = 0.910 Rj. Objects of this size vary by orders of magnitude in their densities, encompassing masses between that of Saturn (0.3 Mj) and stars above the hydrogen-burning limit (~80 Mj). Radial-velocity observations reveal that the companion is unlikely to be a star. The mass posterior is bimodal, indicating a mass of either ~0.24 Mj or ~26 Mj. Continued spectroscopic monitoring should either constrain the mass to be planetary or detect the orbital motion, the latter of which would yield a benchmark long-period brown dwarf with a measured mass, radius, and age.