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We report on the discovery and characterization of the transiting planetK2-39b (EPIC 206247743b). With an orbital period of 4.6 days, it is theshortest-period planet orbiting a subgiant star known to date. Such planets arerare, with only a handful of known cases. The reason for this is poorlyunderstood, but may reflect differences in planet occurrence around therelatively high-mass stars that have been surveyed, or may be the result oftidal destruction of such planets. K2-39 is an evolved star with aspectroscopically derived stellar radius and mass of$3.88^{+0.48}_{-0.42}~\mathrm{R_\odot}$ and$1.53^{+0.13}_{-0.12}~\mathrm{M_\odot}$, respectively, and a very close-intransiting planet, with $a/R_\star = 3.4$. Radial velocity (RV) follow-up usingthe HARPS, FIES and PFS instruments leads to a planetary mass of$50.3^{+9.7}_{-9.4}~\mathrm{M_\oplus}$. In combination with a radiusmeasurement of $8.3 \pm 1.1~\mathrm{R_\oplus}$, this results in a meanplanetary density of $0.50^{+0.29}_{-0.17}$ g~cm$^{-3}$. We furthermorediscover a long-term RV trend, which may be caused by a long-period planet orstellar companion. Because K2-39b has a short orbital period, its existencemakes it seem unlikely that tidal destruction is wholly responsible for thedifferences in planet populations around subgiant and main-sequence stars.Future monitoring of the transits of this system may enable the detection ofperiod decay and constrain the tidal dissipation rates of subgiant stars.

The K2-ESPRINT Project V: a short-period giant planet orbiting a subgiant star