Combined limits on the number of light neutrinos and the top mass from measurement of R=$\sigma$ ( W -> $\ell \nu$) / $\sigma$ ( Z -> $\ell \ell$)

Using the recent measurement of the ratio ofW→ℓv toZ→ℓℓ partial production cross-sections by UA1 and UA2, we derive an upper limit on the number of neutrinos and on the top quark mass, in the framework of the minimal standard model. To reduce the uncertainty on these limits, we use the most recent determination of sin2 θ w and the latest measurements of the structure functions by muon deep-inelastic experiments. No limit onm top is obtained in the case of three neutrino families at the 95% C.L., while a fourth generation is excluded at this confidence level if the top mass is higher thanM w −m b and if there is no new heavy lepton contribution in theW decay. The implications of light fourth generationb' andL are also investigated.