  The $Wtb$ vertex structure and the search for new anomalous couplings is
studied using top quark measurements obtained at the LHC, for a centre-of-mass
energy of 8 TeV. By combining the latest and most precise results on the single
top quark production cross section and the measurements of the $W$-boson
helicity fractions ($F_0$ and $F_L$), it is possible to set new limits, at 95%
CL (confidence level), on the real and imaginary components of the new
couplings. The combination of the LHC observables clearly improves the limits
obtained when using the individual results alone. The updated measurements of
the $W$-boson helicity fractions and the $s+t$-channels electroweak single top
quark production, at the Tevatron, improve the LHC limits, when a world
combination of all observables (LHC+Tevatron) is performed.
