  Methane brown dwarfs, i.e. objects similar to the low mass star Gliese 229B,
are characterized by very unusual colours: J-K<~0 and I-J>~5. An analysis of
the ESO public images, which cover an area of 37 sq. arcmin in the three
filters, yields one clear methane dwarf candidate with J=20.2 and two fainter
sources with J~=23. The resulting observed density of methane dwarfs ranges
from ~100 to ~500 objects per sq. degree with J<23.1, quite high but
compatible, within the errors, with what we derive from simulations of the
stellar population of brown dwarfs in the disk of the Galaxy adopting
reasonable (although still speculative) hypotheses on the evolution of BD
colours with cooling. The predictions presented here can be useful in
constraining the results from future searches of these objects in the infrared.
Deep imaging of several sq. degrees of sky down to J~=22, K~=22 and I>~27 are
necessary to significantly improve the observational database.
