K2-99: a subgiant hosting a transiting warm Jupiter in an eccentric orbit and a long-period companion

We report the discovery from K2 of a transiting planet in an 18.25-d, eccentric (0.19$\pm$ 0.04) orbit around K2-99, an 11th magnitude subgiant in Virgo. We confirm the planetary nature of the companion with radial velocities, and determine that the star is a metal-rich ([Fe/H] = 0.20$\pm$0.05) subgiant, with mass $1.60_{-0.10}^{+0.14}{M}_{\odot }$ and radius $3.1\pm 0.1R_{\odot }$. The planet has a mass of $0.97\pm 0.09M_{\mathrm{J}\mathrm{u}\mathrm{p}}$ and a radius $1.29\pm0.05~R_{\rm Jup}$\mathrm{.}Ameasuredsystemicradialaccelerationof$-2.12\pm0.04~{\rm m s^{-1} d^{-1}}$ offers compelling evidence for the existence of a third body in the system, perhaps a brown dwarf orbiting with a period of several hundred days.