  We formulate a continuum model to study the low-energy electronic structure
of heterostructures formed by graphene on a strong three-dimensional
topological insulator (TI) for the case of both commensurate and incommensurate
stacking. The incommensurability can be due to a twist angle between graphene
and the TI surface or a lattice mismatch between the two systems. We find that
the proximity of the TI induces in graphene a strong enhancement of the
spin-orbit coupling that can be tuned via the twist angle.
