Constraints on singlet right-handed neutrinos coming from the   $Z^0$-width
  We study the constraints on masses and mixing angles imposed by the measured $Z^0$ invisible width, in a model in which a singlet right-handed neutrino mixes with all the Standard Model neutrinos. 
C. O. Escobar, O.L.G. Peres, V. Pleitez and R. Zukanovich Funchal