  We propose a method for recovering the two-dimensional gravitational
potential of galaxy clusters which combines data from weak and strong
gravitational lensing. A first estimate of the potential from weak lensing is
improved at the approximate locations of critical curves. The method can be
fully linearised and does not rely on the existence and identification of
multiple images. We use simulations to show that it recovers the surface-mass
density profiles and distributions very accurately, even if critical curves are
only partially known and if their location is realistically uncertain. We
further describe how arcs at different redshifts can be combined, and how
deviations from weak lensing can be included.
