LSR1610-0040: the first early-type L subdwarf

We report the discovery of LSR1610-0040, a previously unreported r=17.5 star with a very high proper motion mu=1.46"/yr. This very cool star (b-i=6.3) is found to have a peculiar spectrum that does not fit into the standard sequence of late-M dwarfs and L dwarfs. Rather, the spectrum is more typical of an ultra-cool subdwarf, with weak bands of TiO and no detectable VO. But because LSR1610-0040 is so much redder than any other sdM known, and because it does not appear to fit well into the sdM sequence, we propose that LSR1610-0040 be assigned a spectral type sdL, making it the first early-type L subdwarf known. Evidence suggests that LSR1610-0040 is an old, metal-poor star whose mass is just above the limit of hydrogen burning (M ~ 0.08 M_sun).