  An optical lattice with rubidium atoms ($^{85}Rb$) is formed inside a ring
resonator with a finesse of $1.8 \times 10^5$ and a large mode volume of 1.3
$mm^3$. We typically trap several times $10^6$ atoms at densities up to
$10^{12} cm^{-3}$ and temperatures between 25 and 125 $\mu K$. Despite of the
narrow bandwidth (17.3 kHz) of the cavity, heating due to intra--cavity
intensity fluctuations is kept at a low level, such that the time evolution of
the temperature is determined by evaporative cooling.
