  We derive microscopically the Ginzburg-Landau free energy functional for a
superconductor in which the Cooper pairs are formed not only by quasiparticles
from the same band, but also by quasiparticles from different bands. In the
simplest case of an s-wave or d-wave pairing in a two-band system, the order
parameter has three components describing two intraband and one interband pair
condensates. The interband pairing-specific terms in the free energy bring
about some qualitative changes in the phase diagram, for example, time-reversal
symmetry breaking superconducting states are stabilized at low temperatures.
