contribute to the definitive determination of the dominant scenario. The intergalactic medium could also play an important role by accretion of matter as galaxies move along their paths (Kahn & Woltjer 1959; López-Corredoira et al. 2002; Sánchez-Salcedo 2006). Battaner et al. (1990) explained warps by intergalactic magnetic fields, which would produce a direct distortion in the distribution of the gas. The stellar system would also indirectly reach a warped distribution because stars are formed from gas, but this would result in a difference between young and old stars. Colour gradients in warps can thus provide an important clue. Based on this model, gas-poor S0 galaxies should not exhibit warps.

In this paper, we present new, deep NIR and optical observations of 20 edge-on galaxies. NIR wavelengths have rarely been used to date to study warps. They are, however, essential for better tracing the stellar mass distribution, because at infrared (IR) wavelengths ($1 < \lambda <
5 \mu{\rm{m}}$) dust extinction is minimised (the absorption, in magnitudes, in the $K$ filter is 10% of that in the $V$ band, $A_K/A_V \sim 0.1$). NIR emission is also a better tracer of old stellar populations, one of our major objectives in this study. Therefore, 