  We have obtained a low-resolution optical spectrum for one of the faintest
cluster member candidates in Praesepe with the Optical System for Imaging and
low Resolution Integrated Spectroscopy mounted on the 10.4m Gran Telescopio de
Canarias. We confirm spectroscopically the first L dwarf member in Praesepe,
UGCS J084510.66+214817.1. We derived an optical spectral type of L0.3+/-0.4 and
estimated its effective temperature to Teff=2279+/-371 K and a mass of
71.1+/-23.0M_Jup, according to state-of-the-art models, placing it at the
hydrogen-burning boundary. We measured the equivalent width of the
gravity-sensitive sodium doublet at 8182/8194 A, which adds credit to the
membership of this new L dwarf to Praesepe. We also derived a probability of
~20.5% that our candidate would be a field L0 dwarf. We conclude that this
object is likely to be a true member of Praesepe, with evidence of being a
binary system.
