  The elastic properties of two-component bilayer membranes are studied using a
coarse grain model for amphiphilic molecules. The two species of amphiphiles
considered here differ only in their length. Molecular Dynamics simulations are
performed in order to analyze the shape fluctuations of the two-component
bilayer membranes and to determine their bending rigidity. Both the bending
rigidity and its inverse are found to be nonmonotonic functions of the mole
fraction $x_{\rm B}$ of the shorter B-amphiphiles and, thus, do not satisfy a
simple lever rule. The intrinsic area of the bilayer also exhibits a
nonmonotonic dependence on $x_{\rm B}$ and a maximum close to $x_{\rm B} \simeq
1/2$.
