In this work we study theoretically the electronic properties of a sheet of
graphene grown on a periodic heterostructure substrate. We write an effective
Dirac equation, which includes a dependence of both the band gap and the Fermi
velocity on the position, due to the influence of the substrate. This way, both
bandgap and Fermi velocity enter the Dirac equation as operators. The Dirac
equation is solved exactly and we find the superlattice minibands with gaps due
to the breaking of translational symmetry induced by the underlying
heterostructure. The spatial dependence of the Fermi velocity makes the band
gap be indirect, bringing about interesting possibilities for applications in
the design of nanoelectronic devices. In the limit of constant Fermi velocity
we obtain a band structure, with direct band gap, very close to the one
previously found in the literature, obtained using the transfer matrix method