KECK HIRES SPECTRA OF THE BROWN DWARF DENIS-P J1228.2-1547

We report Keck high-resolution echelle spectroscopic observations of t wo very cool field dwarfs discovered by the near-IR photometric survey DENIS. DENIS-P J1228.2-1547 shows a conspicuous LiI resonance line th at confirms it as an incontrovertible brown dwarf(BD). From the presen ce of Li, and its low surface temperature, we estimate from theoretica l models a mass and an age upper limit of similar to 60 jupiters and s imilar to 10(9) years. The other DENIS object shows no detectable LiI line in our data, and thus we infer a mass greater than or equal to 60 jupiters for it. It could be a high-mass ED or very low-mass star. Bo th objects have modest radial velocities that suggest they are kinemat ically young. They show the strongest and broadest resonance line prof iles from low ionization species ever seen. The extreme breadth of the KI lines are good further indicators of the expected high gravity in very low-mass dwarfs. We suggest a new spectral class, ''L'', for obje cts cooler than M-type (as these are) that do not show TiO molecular b ands. The confirmation of at least one field ED in only similar to 1% Of the final DENIS survey is a strong indication of the presence of a numerous population of these objects in the solar neighborhood.