We develop a technique for a joint inversion of two different kinds
of tomographic data. The "objects" to be recovered are connected
just through a topological constraint based on the following weak
assumption linking the two considered data sets: if any
inhomogeneity is present in the investigated volume, it modifies the
values of both the two recorded physical properties. In comparison
with the classical case, where the inversions are performed
independently on each data set and then assembled a
posteriori, the additional a priori information that we use
is the spatial agreement (shape and position) of the anomalies in
the two reconstructed models. The constraint is imposed by
introducing, within the Tikhonov regularization framework, a
stabilizing "joint functional" which couples the two types of data
to be reconstructed.