[en] Let T be a normal contraction (on a complex separable Hilbert space H into itself) with an nth root A such that the defect operator D_A=(1-A*A)^1/2 is of the Hilbert-Schmidt class C₂. Then either A is normal or A is similar to a normal contraction. In the case in which T is hyponormal, Aⁿ=T and D_A is an element of C₂, A is a ''coupling'' of a contraction similar to a normal contraction and a contraction which is the quasi-affine transform of a unilateral shift. These results are applied to prove a (Putnam-Fuglede type) commutatively theorem for operator valued roots of commutative analytic functions and hyponormal contractions T which have an nth root with Hilbert-Schmidt defect operator. 23 refs