  We present Spitzer 7.6-14.5um spectra of ULAS J003402.77-005206.7 and ULAS
J133553.45+113005.2, two T9 dwarfs with the latest spectral types currently
known. We fit synthetic spectra and photometry to the near- through
mid-infrared energy distributions of these dwarfs and that of the T8 dwarf
2MASS J09393548-2448279. We also analyse near-infrared data for another T9,
CFBD J005910.82-011401.3. We find that the ratio of the mid- to near-infrared
fluxes is very sensitive to effective temperature at these low temperatures,
and that the 2.2 and 4.5um fluxes are sensitive to metallicity and gravity;
there is a degeneracy between these parameters. The 4.5 and 10um fluxes are
also sensitive to vertical transport of gas through the atmosphere, which we
find to be significant for these dwarfs. The full near- through mid-infrared
spectral energy distribution allows us to constrain the effective temperature
(K)/gravity (m/s2)/metallicity ([m/H] dex) of ULAS J0034-00 and ULAS J1335+11
to 550-600/ 100-300/ 0.0-0.3 and 500-550/ 100-300/ 0.0-0.3, respectively. These
fits imply low masses and young ages for the dwarfs of 5-20 M(Jup) and 0.1-2
Gyr. The fits to 2MASS J0939-24 are in good agreement with the measured
distance, the observational data, and the earlier T8 near-infrared spectral
type if it is a slightly metal-poor 4-10 Gyr old system consisting of a 500 and
700K, ~25 and ~40 M(Jup), pair, although it is also possible that it is an
identical pair of 600K, 30 M(Jup), dwarfs. As no mid-infrared data are
available for CFBD J0059-01 its properties are less well constrained;
nevertheless it appears to be a 550-600K dwarf with g= 300-2000 m/s2 and [m/H]=
0-0.3 dex. These properties correspond to mass and age ranges of 10-50 M(Jup)
and 0.5-10 Gyr for this dwarf.
