Telling three from four neutrino scenarios

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 $\delta$.