  The structure of few-fermion systems having $1/2$ spin-isospin symmetry is
studied using potential models. The strength and range of the two-body
potentials are fixed to describe low energy observables in the angular momentum
$L=0$ state and spin $S=0,1$ channels of the two-body system. Successively the
strength of the potentials are varied in order to explore energy regions in
which the two-body scattering lengths are close to the unitary limit. This
study is motivated by the fact that in the nuclear system the singlet and
triplet scattering lengths are both large with respect to the range of the
interaction. Accordingly we expect evidence of universal behavior in the three-
and four-nucleon systems that can be observed from the study of correlations
between observables. In particular we concentrate in the behavior of the first
excited state of the three-nucleon system as the system moves away from the
unitary limit. We also analyze the dependence on the range of the three-body
force of some low-energy observables in the three- and four-nucleon systems.
