Mass–radius relation for low-mass EBs. The black points show measurements for stars with masses < 1.5 M_⊙ in detached EBs¹, and the lines show, from top to bottom, the theoretical isochrones of Baraffe et al. (1998, BCAH98) for 1–5, 6.3, 8, 10 Myr and 1 Gyr (brown, blue, purple, lilac, green, grey, ochre, cyan and black, respectively. Y = 0.282, [M/H] = 0, mixing length α = 1.9). The components of the new system presented in this paper are shown by the red diamonds. Note that it lies in a very sparsely populated region of the diagram, making it a valuable test of PMS stellar evolution models. For comparison, we have also labelled the three lowest mass systems known in the Orion Nebula Cluster (see Table 1 for details. For clarity, the higher mass systems are not labelled). Colour versions of all figures are available in the online article.