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Evolutionary models of pre-main sequence stars remain largely uncalibrated,especially for masses below that of the Sun, making each new dynamical mass andradius measurement a valuable test of theoretical models. Stellar massdependent features of star formation (such as disk evolution, planet formation,and even the IMF) are fundamentally tied to these models, which implies asystematic uncertainty that can only be improved with precise measurements ofcalibrator stars. We present the discovery that UScoCTIO 5, a knownspectroscopic binary (P = 34 days, Mtot sin(i) = 0.64 Msun), is an eclipsingsystem with both primary and secondary eclipses apparent in K2 light curvesobtained during Campaign 2. We have simultaneously fit the eclipse profilesfrom the K2 light curves and the existing RV data to demonstrate that UScoCTIO5 consists of a pair of nearly identical M4.5 stars with M_A = 0.329 +/- 0.002Msun, R_A = 0.834 +/- 0.006 Rsun, M_B = 0.317 +/- 0.002 Msun, and R_B = 0.810+/- 0.006 Rsun. The radii are broadly consistent with pre-main sequence agespredicted by stellar evolutionary models, but none agree to within theuncertainties. All models predict systematically incorrect masses at the25--50% level for the HR diagram position of these mid-M dwarfs, suggestingsignificant modifications for any trend of other properties that vary withstellar mass. The form of the discrepancy for most model sets is not that theypredict luminosities that are too low, but rather that they predicttemperatures that are too high, suggesting that the models do not fullyencompass the physics of energy transport (via convection and/or missingopacities) and/or a miscalibration of the SpT-Teff scale. The simplestmodification to the models (changing Teff to match observations) would yield anolder age for this system, in line with the recently proposed older age ofUpper Scorpius (~11 Myr).

The Mass-Radius Relation of Young Stars, I: UScoCTIO 5, An M4.5 Eclipsing Binary in Upper Scorpius Observed By K2