the definition in Sect. \[warppar\]) in any of the four observed bands. Based on this assumption, we find that 13 of our 20 sample galaxies (65%) are warped. This fraction supports previous results based on larger galaxy samples with similar optical characteristics to our sample (Sánchez-Saavedra et al. 1990, 2003; Reshetnikov & Combes 1999). Given the difficulties of observing warps with their line of nodes perpendicular to the line of sight, this high fraction of warped discs indicates that essentially all spirals are warped. Optical warps are less perceptible than HI warps, but at both wavelengths the frequency of warps is the same, i.e., close to 100% (Sancisi 1976; García-Ruiz 2001; García-Ruiz et al. 2002b; van der Kruit 2007).

As expected, the N- and S-like warp frequencies are similar, since these characteristics depend on the observer rather than the galaxy. We observe six galaxies with N-like and four galaxies with S-like warps. The frequency of asymmetric warps is lower: ESO416-G25 exhibits a U-like warp, while ESO240-G11 and ESO555-G36 show L-like warps. Therefore, the most frequent warp morphologies are N- and S-like warps (see also Ann & Park 2006).

The distribution of warped galaxies as a function of morphological type is 