  We prove that the material parameters in a Dirac system with magnetic and
electric potentials are uniquely determined by measurements made on a possibly
small subset of the boundary. The proof is based on a combination of Carleman
estimates for first and second order systems, and involves a reduction of the
boundary measurements to the second order case. For this reduction a certain
amount of decoupling is required. To effectively make use of the decoupling,
the Carleman estimates are established for coefficients which may become
singular in the asymptotic limit.
