  We have obtained low-resolution optical spectra of three faint brown dwarf
candidates in the Pleiades open cluster. The objects observed are Roque 12
(I=18.5), Roque 5 (I=19.7) and Roque 25 (I=21.2). The spectrum of Roque 25 does
not show the strong TiO bandheads that characterize the optical spectra of
M-type stars, but molecular bands of CaH, CrH and VO are clearly present. We
classify Roque 25 as an early L-type brown dwarf. Using current theoretical
evolutionary tracks we estimate that the transition from M-type to L-type in
the Pleiades (age ~ 120 Myr) takes place at Teff ~ 2200 K or M ~ 0.04 Msol.
Roque 25 is a benchmark brown dwarf in the Pleiades because it is the first
known one that belongs to the L-type class. It provides evidence that the IMF
extends down to about 0.035 Msol, and serves as a guide for future deep
searches for even less massive young brown dwarfs.
