Single top production at linear $e^-e^+$ colliders

We study single top production at linear lepton colliders with $\sqrt{s} = 0.5$, 1 and 3 TeV. A preliminary analysis shows that despite the large $t\bar t$ background at $\sqrt{s} = 0.5$ and 1 TeV it is possible to obtain a good sensitivity to the $tbW$ vertex; even more so at 3 TeV when single top becomes the dominant mode of production. Concerning the four dimension six operators involved, two of them simultaneously generate $ttZ$ couplings and their sensitivity decreases with energy. The opposite is true for the other two operators. Single top production at these machines is also useful to probe charged-current four fermion operators $e\nu bt$, some of which are related to $tbW$ operators through the equations of motion.