  Carbon and oxygen abundances of eight late M dwarfs are determined based on
the near IR spectra of medium resolution. Seven objects with T_eff above 2600K
are analyzed with the dust-free models. The M8.5 dwarf 2MASSI J1835379+325954
whose T_eff is 2275K is analyzed by the dusty model, in which the surface
temperature is higher by about 600K due to the blanketing effect of the dust,
and C and O abundances are higher by 0.25 and 0.15dex, respectively, compared
to the analysis by the dust-free model. Once dust forms in the photosphere, the
dust works as a kind of thermostat and temperatures of the surface layers
remain nearly the same as the condensation temperatures of the dust grains. For
this reason, the temperatures of the surface layers of the dusty dwarfs are not
sensitive to the fundamental parameters including T_eff. Also, 2MASS J1835379
+325954 is a rapid rotator, for which its EWs are thought to remain unchanged
by the rotational broadening. This is, however, true only when the true
continuum is well defined. Otherwise, the pseudo-continuum level depends on the
rotational velocity and hence the EWs as well. For this reason, the derived
abundances depend on the rotational velocity assumed: For the values of
V_rot*sin(i)=37.6 and 44.0km/s available in the literature, the derived C and O
abundances differ by 0.23 and 0.14dex, respectively, and we find that the
higher value provides a better account of the observed spectrum. The resulting
C and O abundances in our late M dwarfs show no systematic difference from our
results for the early and middle M dwarfs, and confirm the higher O/C ratio at
the lower metallicity. In late M dwarfs, CO and H2O remain as excellent
abundance indicators of C and O, respectively, except for additional
uncertainty due to the complexity associated with the dust formation in the
latest M dwarfs.
