Off-shell (4,4) supersymmetric sigma models with torsion in harmonic superspace

We present a manifestly supersymmetric off-shell formulation of a wide class of $(4,4)$ $2D$ sigma models with torsion and both commuting and non-commuting left and right complex structures in the harmonic superspace with a double set of $SU(2)$ harmonic variables. The distinguishing features of the relevant superfield action are: (i) in general nonabelian and nonlinear gauge invariance ensuring a correct number of physical degrees of freedom; (ii) an infinite tower of auxiliary fields. This action is derived from the most general one by imposing the integrability condition which follows from the commutativity of the left and right analyticity-preserving harmonic derivatives. For a particular class of such models we explicitly demonstrate the non-commutativity of complex structures on the bosonic target.