A long-period substellar object exhibiting a single transit in Kepler

We report the detection of a single transit-like signal in the Kepler data of the slightly evolved F star KIC4918810. The transit duration is ~45 hours, and while the orbital period ($P\sim10$ 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 $R_P = 0.910$ $R_J$. Objects of this size vary by orders of magnitude in their densities, encompassing masses between that of Saturn ($0.3$ $M_J$) and stars above the hydrogen-burning limit (~80 $M_J$). 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 $M_J$ or ~26 $M_J$. 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.