The general aim of this talk is to show a phenomenon which arise when we deal with the null controllability properties of parabolic coupled systems: minimal time of controllability: 1 Boundary control: The condensation index of the complex sequence of eigenvalues of the corresponding matrix elliptic
operator. 2 Distributed control: The action and the geometric position of the
support of the coupling term when this support does not intersect
the control domain ω
