  In this talk I will consider the possibility of distinguish the usual three
neutrino model from scenarios in which a light sterile neutrino is also
present. I will show that the confusion with the so-called 3+1 scheme can arise
in some particular region of the parameter space whereas it is essentially
absent for the 2+2 scheme.
  Then I will discuss the ambiguities in the determination of the CP-violating
phase $\delta$.
