We present high-, mid-, and low-resolution spectroscopy of the Ðeld brown dwarf candidate PC 0025]0447 (M9.5) spanning a 4 yr baseline (1994È1998). The strength of the emission lines and the amount of optical veiling are very variable. Our spectra taken at an epoch of low veiling allow us to detect the lithium resonance line, which was not detected when the veiling was high. The presence of lithium proves that PC 0025]0447 is a substellar object less massive than and younger than D0.06 M _ D1

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... r. We also present mid-resolution spectra of three brown dwarf candidates in star-forming regions, namely o Oph 162349.8[242601 (M8.5), V410 Tau X3 (M6.5), and V410 Tau X6 (M6). Lithium is detected in all of them, supporting their very young age. Of these three objects, only the one in o Oph is sufficiently cool to warrant an unambiguous substellar status. The spectroscopic characteristics that PC 0025]0447 and o Oph 162349.8[242601 have in common are moderately strong Li I lines (pseudoÈ equivalent width D 1 persistent Ha emission with equivalent width greater than and weaker A ), 50 A , K I and Na I lines than Ðeld dwarfs of the same spectral type.