  We consider structure and emission properties of a pair plasma fireball that
cools due to radiation. At temperatures $T \geq 0.5 m_e c^2$ the cooling takes
a form of clearly defined cooling wave, whereby the temperature and pair
density experience a sharp drop within a narrow region. The surface
temperature, corresponding to the location where the optical depth to infinity
reaches unity, never falls much below $0.1 m_e c^2 \approx 50$ keV. The
propagation velocity of the cooling wave is much smaller than the speed of
light and decreases with increasing bulk temperature.
