  We present the discovery of CoRoT 223992193, a double-lined, detached
eclipsing binary, comprising two pre-main sequence M dwarfs, discovered by the
CoRoT space mission during a 23-day observation of the 3 Myr old NGC 2264
star-forming region. Using multi-epoch optical and near-IR follow-up
spectroscopy with FLAMES on the Very Large Telescope and ISIS on the William
Herschel Telescope we obtain a full orbital solution and derive the fundamental
parameters of both stars by modelling the light curve and radial velocity data.
The orbit is circular and has a period of $3.8745745 \pm 0.0000014$ days. The
masses and radii of the two stars are $0.67 \pm 0.01$ and $0.495 \pm 0.007$
$M_{\odot}$ and $1.30 \pm 0.04$ and $1.11 ~^{+0.04}_{-0.05}$ $R_{\odot}$,
respectively. This system is a useful test of evolutionary models of young
low-mass stars, as it lies in a region of parameter space where observational
constraints are scarce; comparison with these models indicates an apparent age
of $\sim$3.5-6 Myr. The systemic velocity is within $1\sigma$ of the cluster
value which, along with the presence of lithium absorption, strongly indicates
cluster membership. The CoRoT light curve also contains large-amplitude,
rapidly evolving out-of-eclipse variations, which are difficult to explain
using starspots alone. The system's spectral energy distribution reveals a
mid-infrared excess, which we model as thermal emission from a small amount of
dust located in the inner cavity of a circumbinary disk. In turn, this opens up
the possibility that some of the out-of-eclipse variability could be due to
occultations of the central stars by material located at the inner edge or in
the central cavity of the circumbinary disk.
