Discovery of an ultra-cool subdwarf: LSR1425+7102, first star with spectral type sdM8.0

We report the discovery of the coolest subdwarf reported to date. The star LSR1425+7102 was discovered in our survey for faint high proper motion stars in the northern sky. Follow-up spectroscopy revealed the star to be a very red object with the characteristic signature of M subdwarfs: strong CaH bands but relatively weaker TiO bands. The CaH molecular band at $\approx\lambda$6900\AA\ is particularly strong. By extrapolating the empirical relationship between the strength of the CaH molecular band and the subdwarf subtype, we conclude that LSR1425+7102 is the first star to be discovered with spectral type sdM8.0.