  We study the temperature dependence of optical properties of dilute gases of
alkali-metal atoms in the state with Bose-Einstein condensates. The description
is constructed in the framework of the microscopic approach that is based on
the Green-functions formalism. We find the expressions for the scalar Green
functions describing a linear response of a condensed gas to a weak external
electromagnetic field (laser). It is shown that these functions depend on the
temperature, other physical properties of a system, and on the frequency
detuning of a laser. We compare the relative contributions of the condensate
and non-condensate particles in the system response. The influence of the
temperature effects is studied by the example of two- and three-level systems.
We show that in these cases, which are most commonly realized in the present
experiments, the group velocity and the absorption rate of pulses practically
do not depend on the gas temperature in the region from the absolute zero to
the critical temperature. We discuss also the cases when the temperature
effects can play a significant role in the phenomenon of slowing of
electromagnetic pulses in a gas of alkali-metal atoms with Bose-Einstein
condensates.
