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Despite more than 20 years since the discovery of the first gas giant planetwith an anomalously large radius, the mechanism for planet inflation remainsunknown. Here, we report the discovery of EPIC228754001.01, an inflated gasgiant planet found with the NASA K2 Mission, and a revised mass for anotherinflated planet, K2-97b. These planets reside on ~9 day orbits around hoststars which recently evolved into red giants. We constrain the irradiationhistory of these planets using models constrained by asteroseismology andKeck/HIRES spectroscopy and radial velocity measurements. We measure planetradii of 1.31 +\- 0.11 Rjup and and 1.30 +\- 0.07 Rjup, respectively. Theseradii are typical for planets receiving the current irradiation, but not theformer, zero age main sequence irradiation of these planets. This suggests thatthe current sizes of these planets are directly correlated to their currentirradiation. Our precise constraints of the masses and radii of the stars andplanets in these systems allow us to constrain the planetary heating efficiencyof both systems as 0.03% +0.03%/-0.02%. These results are consistent with aplanet re-inflation scenario, but suggest the efficiency of planet re-inflationmay be lower than previously theorized. Finally, we discuss the agreementwithin 10% of stellar masses and radii, and planet masses, radii, and orbitalperiods of both systems and speculate that this may be due to selection bias insearching for planets around evolved stars.

Seeing double with K2: Testing re-inflation with two remarkably similar planets around red giant branch stars