  Cool subdwarfs are metal-poor low-mass stars that formed during the early
stages of the evolution of our Galaxy. Because they are relatively rare in the
vicinity of the Sun, we know of few cool subdwarfs in the solar neighbourhood,
and none with both the mass and the radius accurately determined. This hampers
our understanding of stars at the low-mass end of the main-sequence. Here we
report the discovery of SDSSJ235524.29+044855.7 as an eclipsing binary
containing a cool subdwarf star, with a white dwarf companion. From the
light-curve and the radial-velocity curve of the binary we determine the mass
and the radius of the cool subdwarf and we derive its effective temperature and
luminosity by analysing its spectral energy distribution. Our results validate
the theoretical mass-radius-effective temperature-luminosity relations for
low-mass low-metallicity stars.
