[en] Motivated by recent JET results showing an enhancement in the lower hybrid current drive (LHCD) efficiency in ICRF heated plasmas, the authors have been studying the interaction of lower hybrid generated electron tails with fast Alfven waves (FAW) and ion-Bernstein waves (IBW). The IBW's are generated by mode conversion, at the hybrid resonance layers inside the plasma, of the externally launched FAW's. The numerical solutions of the Fokker-Planck equation using CQL3D show that, for the powers used in JET, the FAW cannot significantly modify the LH electron tail and, consequently, does not play a role in the enhancement of the LHCD efficiency. Similar analysis for IBW's shows that these waves play an important role in the enhancement of the current drive efficiency. Calculations show that for small k_parallel's about 20% of the FAW power is mode converted to the IBW's. Previous analysis of the propagation of IBW's has shown that the electric field amplitudes are considerably enhanced as IBW propagate in toroidal plasmas. This compensates for the possibly small fraction of the total input power that mode converts to IBW's. Furthermore, IBW's interact with electrons off-axis through Landau damping on flux surfaces where LHW's generate currents. FAW's only interact with electrons near the ion cyclotron or ion-ion hybrid resonance layers which, in the case of JET, may be near the axis of the plasma while the LH waves damp well off-axis. So IBW's are more likely to enhance the LHCD efficiency. In fact, the conditions for which JET shows an enhancement in LHCD efficiency are those for which mode conversion to IBW's occurs. The authors present detailed analytical and numerical results in support of the statements made above