longer-wavelength $K$ filter ($\lambda_{\rm eff} = 2.20 \mu$m) because the sky brightness in $K_{\rm{s}}$ is lower than in the $K$ band (for instance, at UKIRT, Hawaii, the sky brightness is 13.5 and 13.0 mag arcsec$^{-2}$ in $K_{\rm{s}}$ and $K$, respectively[^1]) and $K_{\rm{short}}$ is almost as little affected by dust as the $K$ band, $A_{K_{\rm s}} \sim 0.11$ mag airmass$^{-1}$; and

-   **Optical** wavelengths. We obtained our observations using two telescopes of the European Southern Observatory (ESO) at La Silla, Chile, including the 1.54 m Danish telescope (equipped with a 1081$\times$1040-pixel TEK CCD with $0.36 \arcsec$ pixel$^{-1}$) and the 0.92 m Dutch telescope (equipped with a 512$\times$512-pixel TEK CCD with $0.44 \arcsec$ pixel$^{-1}$), both in the standard Johnson $B$ and $V$ and Thuan & Gunn (1976) $i$ filters.

The observations and optical data-reduction techniques on which the results presented in this paper are based were described in detail in de Grijs (1998). Here, we focus on the reduction of the IR data.

To study stellar warps in disc galaxies, we need to obtain reliable data at the very low surface brightnesses in their outermost regions. Our observational data reached surface brightnesses fainter than 26 mag arcsec$^{-2}$ in the $B$ band. This 