We will discuss toroidal compactifications of the bosonic and heterotic strings, focusing on the phenomenon of symmetry enhancement arising at self-dual points in moduli space.

I will present the effective action describing the low energy effective theory close to the points of enhancement, obtained from string theory scattering amplitudes, show how symmetry breaking arises naturally as a Higgs mechanism, and show that this action can be obtained from a suitable (twisted) Sherk-Schwarz reduction of double field theory.