\setstretch 1.1 We present the characterization of CRTS J055255.7 - 004426 ( =THOR 42 ) , a young eclipsing binary comprising two pre-main sequence M dwarfs ( combined spectral type M3.5 ) .
This nearby ( 103 pc ) , short-period ( 0.859 d ) system was recently proposed as a member of the \sim 24 Myr-old 32 Orionis Moving Group .
Using ground- and space-based photometry in combination with medium- and high-resolution spectroscopy , we model the light and radial velocity curves to derive precise system parameters .
The resulting component masses and radii are 0.497 \pm 0.005 and 0.205 \pm 0.002 M _ { \odot } , and 0.659 \pm 0.003 and 0.424 \pm 0.002 R _ { \odot } , respectively .
With mass and radius uncertainties of \sim 1 per cent and \sim 0.5 per cent , respectively , THOR 42 is one of the most precisely characterized pre-main sequence eclipsing binaries known .
Its systemic velocity , parallax , proper motion , colour–magnitude diagram placement and enlarged radii are all consistent with membership in the 32 Ori Group .
The system provides a unique opportunity to test pre-main sequence evolutionary models at an age and mass range not well constrained by observation .
From the radius and mass measurements we derive ages of 22–26 Myr using standard ( non-magnetic ) models , in excellent agreement with the age of the group .
However , none of the models can simultaneously reproduce the observed mass , radius , temperature and luminosity of the coeval components .
In particular , their H–R diagram ages are 2–4 times younger and we infer masses \sim 50 per cent smaller than the dynamical values .