Accurate mass and radius determinations of a cool subdwarf in an eclipsing binary

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 for which both the mass and the radius are 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 relations between mass, radius, effective temperature and luminosity for low-mass, low-metallicity stars.