The stability of standing waves with small group velocity

We determine the modulational stability of standing waves with small group velocity in quasi-one-dimensional systems slightly above the threshold of a supercritical Hopf bifurcation. The stability limits are given by two different long-wavelength destabilization mechanisms and generically also a short-wavelength destabilization. The Eckhaus parabola is shifted off-center and can be convex from below or above. For non-zero velocity the Newell criterion, which near the cross-over from standing to traveling waves becomes a rather weak condition, does not determine the destabilisation of all standing waves in one dimension. The cross-over to the non-local and the hyperbolic equations that are asymptotically valid near threshold is discussed in detail. Close to the transition from standing to traveling waves complex dynamics can arise due to the competition of counter-propagating waves and the wave number selection by sources. Our results yield necessary conditions for the stability of traveling rectangles in quasi-two-dimensional systems with axial anisotropy and form a starting point for understanding the spatio-temporal chaos of traveling oblique rolls observed in electroconvection of nematic liquid crystals.