  We consider systems of reaction-diffusion equations coupled in zero order
terms, with general homogeneous boundary conditions in domains with a
particular geometry (annular type domains). We establish Lipschitz stability
estimates in L^2 norm for the source in terms of the solution and/or its normal
derivative on a connected component of the boundary. The main tools are
represented by: appropriate Carleman estimates in L^2 norms, with boundary
observations, and positivity improving properties for the solutions to
parabolic equations and systems.
