  We upgrade the study of the physical reach of a Neutrino Factory considering
the possibility to distinguish a three (active) neutrino oscillation scenario
from the scenario in which a light sterile neutrino is also present. The
distinction is easily performed in the so--called 2+2 scheme, but also in the
more problematic 3+1 scheme it can be attained in some regions of the parameter
space. We also discuss the CP violating phase determination, showing that the
effects of a large phase in the three--neutrino theory cannot be reproduced in
a four--neutrino, CP conserving, model.
