  We detail the construction of the exceptional sigma model, which describes a
string propagating in the "extended spacetime" of exceptional field theory.
This is to U-duality as the doubled sigma model is to T-duality. Symmetry
specifies the Weyl-invariant Lagrangian uniquely and we show how it reduces to
the correct 10-dimensional string Lagrangians. We also consider the inclusion
of a Fradkin-Tseytlin (or generalised dilaton) coupling as well as a
reformulation with dynamical tension.
