Single W production is studied in the data recorded with the ALEPH detector
at LEP at centre-of-mass energies between 161 and 183 GeV. The cross secti
on is measured to be sigma(w) = 0.41 +/- 0.17(stat.) +/- 0.04(syst.) pb at
183 GeV, consistent with the Standard Model expectation. Limits on non-stan
dard WW gamma couplings are deduced as -1.6 < kappa gamma < 1.5 (lambda(gam
ma) = 0) and -1.6 < lambda(gamma) < 1.6 (kappa(gamma) = 1) at 95% C.L. A se
arch for effectively invisible decays of the W boson in W pair production i
s performed, leading to an upper Limit on the branching ratio of 1.3% (Gamm
a(inv) = 27 MeV) at 95% C.L. (C) 1999 Elsevier Science B.V. All rights rese
rved.