  We present intermediate and low resolution optical spectroscopy (650-915 nm)
of seven faint, very red objects (20 > I >= 17.8, I-Z >= 0.5) discovered in a
CCD-based IZ survey covering an area of 1 sq. deg in the central region of the
Pleiades open cluster. The observed spectra show that these objects are very
cool dwarfs, having spectral types in the range M6-M9. Five out of the seven
objects can be considered as Pleiades members on the basis of their radial
velocities, Halpha emission and other gravity sensitive atomic features like
the NaI doublet at 818.3 and 819.5 nm. According to current evolutionary models
the masses of these new objects range from roughly 80 MJup for the hottest in
the sample down to 45 MJup for Roque 4, the coolest and faintest confirmed
member. These observations prove that the cloud fragmentation process extends
well into the brown dwarf realm, suggesting a rise in the initial mass function
below the substellar limit.
