Radial velocity follow up of Wolf 503

In addition, we present improved orbital and transit parameters by analyzing previously unused short-cadence K2 campaign 17 photometry and conduct a joint radial-velocity-transit fit to constrain the eccentricity at 0.41{+/-}0.05. The addition of a transit observation by Spitzer also allows us to refine the orbital ephemeris in anticipation of further follow-up.

Our mass determination, 6.26_-0.70_^+0.69^M{Earth} , in combination with the updated radius measurements, gives Wolf503b a bulk density of {rho}=2.92_-0.44_^+0.50^g/cm^3^. Using interior composition models, we find this density is consistent with an Earth-like core with either a substantial H_2_O mass fraction (45_-16_^+19^%) or a modest H/He envelope (0.5%{+/-}0.3%). The low H/He mass fraction, along with the old age of Wolf503 (11{+/-}2Gyr), makes this sub-Neptune an opportune subject for testing theories of XUV-driven mass loss while the brightness of its host (J=8.3mag) makes it an attractive target for transmission spectroscopy.