Tri-vector symmetry of 11 dimensional supergravity

Kaluza-Klein reductions of 11-dimensional supergravity lead to exceptional global symmetries in lower dimensions. Certain non-geometric elements of these symmetries, parameterized by a tri-vector $\gamma$, are not inherited from the higher-dimensional local symmetries, but represent instead a symmetry enhancement produced by the isometries of the background. Here, we demonstrate how to realize this enhancement in 11 dimensions, as a symmetry principle with constrained parameters. We show that $\gamma$ transformations exchange the equations of motion of the metric and the three-form with their Bianchi identities, in a closed form, structuring them into tri-vector multiplets. Implementing this principle as an off-shell symmetry of the theory requires the introduction of a hierarchy of dual fields, including a six-form and a dual graviton in the initial levels.