  We extend standard k.p theory to take into account periodic perturbations
which are rapidly oscillating with a wavelength of a few lattice constants. Our
general formalism allows us to explicitly consider the Bragg reflections due to
the perturbation-induced periodicity. As an example we calculate the effective
masses in the lowest two conduction bands of spontaneously ordered GaInP_2 as a
function of the degree of ordering. Comparison of our results for the lowest
conduction band to available experimental data and to first principle
calculations shows good agreement.
