in the disc, either triggered by an interaction with another galaxy in the past or caused by the disc being embedded in a nonspherical halo of dark matter. Sparke & Casertano (1988) showed that some discrete oscillation modes can survive. This theory is not without problems, however; for instance, Binney (1991) showed that the halo should then respond to its misalignment with the disc, thus destroying the warp in a few orbital periods. Alternatively, a nonlinear coupling between a spiral wave and two warp waves could create a warp (Masset & Tagger 1997). The efficiency of this mechanism is too low in the stellar disc except at its outer edge (near a Lindblad resonance), where the spiral wave slows down and is coupled efficiently to the warp waves. At those radii, the energy of the spiral arm can almost be completely converted into transmitted and reflected warp waves, which can be observed as corrugations in the disc.

Warping could thus be a natural response of the outer disc to a series of stimulations, and it seems that the responsible mechanism does not necessarily have to be the same in each galaxy. In any case, observations of large edge-on galaxies could 