Discovery of the lowest mass brown dwarf in the Pleiades

We have imaged the Pleiades open cluster at I and Z in a search for low-mass stars and brown dwarfs. One very red object, which we have called PIZ1, at I=19.64, I —Z = 1.33 has been detected within an area of 100 arcmin2. Follow-up infrared photometry verifies that this object is extremely red with a K magnitude of 15.5. We have also obtained a spectrum which exhibits the spectral features indicative of an extremely cool M dwarf. We estimate the effective temperature and mass of PIZ 1 to be 2300 K and 0.048 M⊙, respectively.