[en] Magnetic and neutron diffraction measurements are reported. These compounds which order magnetically for 0 < approx. c <= 1, are helimagnetic at least for c > approx. 0.40. Cubic symmetry crystal field effects on Tb³⁺ yield to a 10 K overall splitting a GAMMA₃ non magnetic doublet being the ground state at 0.7 K below a GAMMA₅¹ triplet. (orig./SMV)