The magnetic phase transition 3 mL' .tautm. m with the Curie temp. Tc .simeq. 30K was studied by optical domain anal. and birefringence measurements using polarized light microscopy. The domain structure is consistent with .hivin.43mL'Fm with 24 partially ferromagnetic/fully ferroelec./fully ferroelastic orientation states. The weakly ferromagnetic m phase possesses a spontaneous magnetization perpendicular to the (110)cub planes (mirror m). A coupling effect between the ferromagnetic, ferroelec. and ferroelastic domains was obsd. and detected. The spontaneous birefringence of magnetic and possibly coupled structural origin vanishes above Tc and exhibits a 2nd order character of transition.