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 10.4 m 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 \pm 0.4 and estimated its effective temperature to T _ { eff } = 2279 \pm 371 K and a mass of 71.1 \pm 23.0 M _ { 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 8182/8194Å , which adds credit to the membership of this new L dwarf to Praesepe .
We also derived a probability of \sim 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 .