We report the discovery of EPIC 219388192 b , a transiting brown dwarf in a 5.3-day orbit around a member star of Ruprecht-147 , the oldest nearby open cluster association , which was photometrically monitored by K2 during its Campaign 7 .
We combine the K2 time-series data with ground-based adaptive optics imaging and high resolution spectroscopy to rule out false positive scenarios and determine the main parameters of the system .
EPIC 219388192 b has a radius of R _ { \mathrm { b } } = 0.937 \pm 0.042 { R _ { \mathrm { Jup } } } and mass of M _ { \mathrm { b } } = 36.50 \pm 0.09 { M _ { \mathrm { Jup } } } , yielding a mean density of 59.0 \pm 8.1 { g cm ^ { -3 } } .
The host star is nearly a Solar twin with mass M _ { \star } = 0.99 \pm 0.05 { M _ { \odot } } , radius R _ { \star } = 1.01 \pm 0.04 { R _ { \odot } } , effective temperature T _ { eff } = 5850 \pm 85 K and iron abundance [ Fe/H ] = 0.03 \pm 0.08 dex .
Its age , spectroscopic distance , and reddening are consistent with those of Ruprecht-147 , corroborating its cluster membership .
EPIC 219388192 b is the first brown dwarf with precise determinations of mass , radius and age , and serves as benchmark for evolutionary models in the sub-stellar regime .