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We propose to record the first spectrum of a brown dwarf (Gliese 229B)
longward of 5 microns (4.9-12.2 microns), using the ISOCAM instrument with
the CVF. Gliese 229B, a weak companion to the nearby M1 V star Gl229A, is
the only unambiguous brown dwarf discovered to date. Its spectrum has been
explored only in the 1- to 2.5-micron range, revealing spectral signatures
from methane and water vapour. In the observations proposed here, we expect
to detect strong absorption features from these two compounds between 5.2
and 8.2 microns and to make the first detection of ammonia from its nu_2 band
at 10.5 microns. These observations will provide unique information on the
thermal structure of Gliese 229B, on its elemental C and O abundances, and
ultimately on its age, mass, and evolution. This is a new proposal since
the discovery of Gliese 229B occured after the initial call for ISO proposals.