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Using spectroscopic radial velocities with the APOGEE instrument and Gaiadistance estimates, we demonstrate that Kepler-503b, currently considered avalidated Kepler planet, is in fact a brown-dwarf/low-mass star in a nearlycircular 7.2-day orbit around a subgiant star. Using a mass estimate for theprimary star derived from stellar models, we derive a companion mass and radiusof $0.075\pm0.003 \ M_{\odot}$ ($78.6\pm3.1 \ M_{Jup}$) and$0.099^{+0.006}_{-0.004}\ R_{\odot}$ ($0.96^{+0.06}_{-0.04}\ R_{Jup}$),respectively. Assuming the system is coeval, the evolutionary state of theprimary indicates the age is $\sim6.7$ Gyr. Kepler-503b sits right at thehydrogen burning mass limit, straddling the boundary between brown dwarfs andvery low-mass stars. More precise radial velocities and secondary eclipsespectroscopy with James Webb Space Telescope will provide improved measurementsof the physical parameters and age of this important system to better constrainand understand the physics of these objects and their spectra. This systememphasizes the value of radial velocity observations to distinguish a genuineplanet from astrophysical false positives, and is the first result from theSDSS-IV monitoring of Kepler planet candidates with the multi-object APOGEEinstrument.

Kepler-503b: An Object at the Hydrogen Burning Mass Limit Orbiting a Subgiant Star