Four species neutrino oscillations at $ u$-Factory: sensitivity and CP-violation

The prospects of measuring the leptonic angles and CP-odd phases at a {em neutrino factory} are discussed in the scenario of three active plus one sterile neutrino. We consider the $ u_mu raw u_e$ LSND signal. Its associated large mass difference leads to observable neutrino oscillations at short ($sim 1$ km) baseline experiments. Sensitivities to the leptonic angles down to $10^{-3}$ can be easily achieved with a 1 Ton detector. Longer baseline experiments ($sim 100$ km) with a 1 Kton detector can provide very clean tests of CP-violation especially through tau lepton detection.