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Physics of W bosons at LEP2

After the first observations of W bosons in leptonic interactions, about 4000 WW candidate events per experiment have been collected at LEP2. This data allows the measurement of the WW production cross section at different centre-of-mass energies, as well as W decay branching fractions. The W hadronic branching fraction can be converted into a test of the unitarity of the CKM matrix, or into an indirect determination of the matrix element $|V_{cs}|$. A more direct measurement coming from charm tagging is also performed. The W mass has been measured via the cross section (in the threshold region) and the direct reconstruction of the W decay products, using different techniques to account for the distortions due to experimental effects. The main systematic error to the mass reconstruction in the fully hadronic channel comes from QCD effects like Color reconnections and Bose-Einstein correlations, extensively studied in WW events. In $e^+e^-$ collisions W pairs can be produced in s-channel via a three vector boson vertex, so a direct study of the trilinear gauge boson couplings is possible. Modification of WW cross section and distributions of W production and decay angles would be an indication of non-standard couplings, thus a first hint for the presence of new physics.