  We consider a transmission wave equation in two embedded domains in $R^2$,
where the speed is $a1 > 0$ in the inner domain and $a2 > 0$ in the outer
domain. We prove a global Carleman inequality for this problem under the
hypothesis that the inner domain is strictly convex and $a1 > a2$ . As a
consequence of this inequality, uniqueness and Lip- schitz stability are
obtained for the inverse problem of retrieving a stationary potential for the
wave equation with Dirichlet data and discontinuous principal coefficient from
a single time-dependent Neumann boundary measurement.
