warped S0 galaxies. Here, we therefore pay special attention to S0 galaxies. Reshetnikov et al. (2002) find that warps were also common in the past, with even greater magnitudes at $z \approx 1$.

Edge-on galaxies are usually chosen to study warps, although dynamical warps in the HI distribution of galaxies of intermediate inclination have also been reported, e.g., in M83 (Rogstad et al. 1974), NGC 5033, NGC 5055, NGC2841, and NGC7331 (Bosma 1978), M31 (Brinks & Burton 1984), and M33 (Corbelli et al. 1989). García-Ruiz et al. (2002b) analysed HI observations and find that all edge-on galaxies that have an extended HI disc (with respect to the optical component) are warped.

Ann & Park (2006) find that 73% of the 325 galaxies in their well-defined sample of almost perfectly edge-on galaxies ($a/b > 9.5$, where $a/b$ is the major-to-minor axis ratio at $\mu_{B} = 25$ mag arcsec$^{-2}$) exhibit optical warps. Recently, Reylé et al. (2009) have found that a warp is present in the stellar, dust, and gas discs of the Milky Way, which are all asymmetric and characterised by a similar line of nodes. The Milky Way’s HI gas warp is the strongest, followed by the dust warp, while 