  Thermally excited capillary waves at fluid interfaces in binary liquid
mixtures exhibit simultaneously both density and composition fluctuations.
Based on a density functional theory for inhomogeneous binary liquid mixtures
we derive an effective wavelength dependent Hamiltonian for fluid interfaces in
these systems beyond the standard capillary-wave model. Explicit expressions
are obtained for the surface tension, the bending rigidities, and the coupling
constants of compositional capillary waves in terms of the profiles of the two
number densities characterizing the mixture. These results lead to predictions
for grazing-incidence x-ray scattering experiments at such interfaces.
