  We prove existence and stability results for a two-parameter family of
solitary-wave solutions to a system in which an equation of nonlinear
Schr\"odinger type is coupled to an equation of Korteweg-de Vries type. Such
systems model interactions between short and long dispersive waves. The results
extend earlier results of Angulo, Albert and Angulo, and Chen. Our proof
involves the characterization of solitary-wave solutions as minimizers of an
energy functional subject to two constraints. To establish the precompactness
of minimizing sequences via concentrated compactness, we establish the
sub-additivity of the problem with respect to both constraint variables
jointly.
