The constraints on the number of neutrinos generations,
g'1 coupling and
Z - Z′ mixing angle through the invisible width method, and in the framework of a U(1)
B-L model are obtained. Based on the experimental value reported by the LEP for the rate
, we obtained a bound on the
g'1 coupling,
. In addition, we derive 90% C.L. bounds on the
Z - Z′ mixing angle
, improving the existing bounds by one order of magnitude.