  We use the Gross-Pitaevskii equation to determine the spatial structure of
the condensate density of interacting bosons whose energy dispersion epsilon_k
has two degenerate minima at finite wave-vectors q. We show that in general the
Fourier transform of the condensate density has finite amplitudes for all
integer multiples of q. If the interaction is such that many Fourier components
contribute, the Bose condensate is localized at the sites of a one-dimensional
lattice with spacing 2pi/q; in this case Bose-Einstein condensation resembles
the transition from a liquid to a crystalline solid. We use our results to
investigate the spatial structure of the Bose condensate formed by magnons in
thin films of ferromagnets with dipole-dipole interactions.
