  We report the discovery of a low-mass (0.26 +/- 0.02, 0.15 +/- 0.01 Msol)
pre-main-sequence eclipsing binary with a 5.3 day orbital period. JW 380 was
detected as part of a high-cadence time-resolved photometric survey (the
Monitor project) using the 2.5m Isaac Newton Telescope and Wide Field Camera
for a survey of a single field in the Orion Nebula Cluster (ONC) region in V
and i bands. The star is assigned a 99 per cent membership probability from
proper motion measurements, and radial velocity observations indicate a
systemic velocity within 1 sigma of that of the ONC. Modelling of the combined
light and radial velocity curves of the system gave stellar radii of 1.19 +0.04
-0.18 Rsol and 0.90 +0.17 -0.03 Rsol for the primary and secondary, with a
significant third light contribution which is also visible as a third peak in
the cross-correlation functions used to derive radial velocities. The masses
and radii appear to be consistent with stellar models for 2-3 Myr age from
several authors, within the present observational errors. These observations
probe an important region of mass-radius parameter space, where there are
currently only a handful of known pre-main-sequence eclipsing binary systems
with precise measurements available in the literature.
