  We present a theoretical study of the phase diagram and the structure of a
fluid adsorbed in high-porosity aerogels by means of an integral-equation
approach combined with the replica formalism. To simulate a realistic gel
environment, we use an aerogel structure factor obtained from an off-lattice
diffusion-limited cluster-cluster aggregation process. The predictions of the
theory are in qualitative agreement with the experimental results, showing a
substantial narrowing of the gas-liquid coexistence curve (compared to that of
the bulk fluid), associated with weak changes in the critical density and
temperature. The influence of the aerogel structure (nontrivial short-range
correlations due to connectedness, long-range fractal behavior of the silica
strands) is shown to be important at low fluid densities.
