  The two scale convergence of the solution to a Robin's type-like problem of a
stationary diffusion problem in a periodically perforated domain is
investigated. It is shown that the Robin's problem converges to a problem
associated to a new operator which is the sum of a standard homogenized
operator plus an extra first order "strange" term; its appearance is due to the
non-symmetry of the diffusion matrix and to the non rescaled resistivity.
