Field brown dwarfs found by DENIS

We present three candidate field brown dwarfs, identified by the DENIS survey. This on-going three colour survey of the southern sky has, as of July 1997, covered over 30% of the southern hemisphere in the I, J and K_short passbands. The enormous sampled volume makes DENIS ideal for detecting brown dwarfs. The present results are based on preliminary processing of about 230 square degrees of DENIS data -- a small fraction of the existing sky coverage. This reveals a sizeable population of very cold dwarfs, three of which are at least as cool as GD 165B. Infrared spectra confirm their dwarf status, and provide effective temperature information: one object (DENIS-P J1058.7-1548) is comparable in temperature to GD 165B and a second (DENIS-P J1228.2-1547) is slightly cooler, while a third (DENIS-P J0205.4-1159) is significantly so -- though not as cold as Gl 229B. The infrared spectrum of DENIS-P J0205.4-1159 shows evidence for a methane absorption band, which implies an effective temperature much below the stellar limit. Lastly, recent detections of lithium in DENIS-P J1228.2-1547 have proven it to have a substellar mass. This makes it (together with the recently discovered object Kelu-1) the first bona-fide isolated field brown dwarf. Partly based on observations obtained at the European Southern Observatory