  A theoretical study of the improvement of the Electron Cyclotron Current
Drive (ECCD) efficiency in regimes where most of the current is driven by Lower
Hybrid (LH) waves is presented. A perturbation technique is employed to solve
the adjoint equation and derive the response function including both
collisional and LH effects in the limit where the former dominate. An
alternative treatment of the problem, involving a numerical solution of the
Langevin equations is proposed to gain insight into the current drive mechanism
and confirm the obtained results. The existence of a cross-effect between the
two waves is demonstrated and the conditions for the synergy, i.e. significant
enhancement of the ECCD efficiency in the presence of LH power, are identified.
