  Ginzburg-Landau-de Gennes -type covariant theories are extensively used in
connection with twist grain boundary (TGB) phases of chiral smectogens. We
analyze the stability conditions for the linear, covariant elasticity theory of
smectic-C liquid crystals in the context of achiral materials, and predict an
equilibrium modulated structure with an oblique wavevector. We suggest that a
previous experimental observation of stripes in smectic-C is consistent with
the predicted structure.
