  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 $R_\mathrm{b}$=$0.937\pm0.042$~$\mathrm{R_{Jup}}$ and mass of
$M_\mathrm{b}$=$36.50\pm0.09$~$\mathrm{M_{Jup}}$, yielding a mean density of
$59.0\pm8.1$~$\mathrm{g\,cm^{-3}}$. The host star is nearly a Solar twin with
mass $M_\star$=$0.99\pm0.05$~$\mathrm{M_{\odot}}$, radius
$R_\star$=$1.01\pm0.04$~$\mathrm{R_{\odot}}$, effective temperature
$\mathrm{T_{eff}}$=$5850\pm85$~K and iron abundance [Fe/H]=$0.03\pm0.08$~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.
