the dust lane). This method was applied mainly at blue wavelengths to compensate for background stars.

3.  Third, we fitted Gaussian profiles. We fitted each vertical trace (parallel to the minor axis of the galaxy) in the selected zone to a Gaussian function. We considered this the best approach, because the contribution of the dust lane is less important in optical data than when using the other two methods. Gaussians can be matched to vertical profiles that contain a dip caused by a dust lane without problems in almost perfectly edge-on galaxies.

Warp curves from the Gaussian fit
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As a first step, we rotated our galaxies (to align their major axes with the horizontal axis) using an iterative method until the position angle (PA) had been determined to an accuracy of approximately 0.05 degrees. We used the PA from the ESO-LV Catalogue as a starting point; we then iteratively fitted a straight line to the central region of the galaxy. Our results are shown in Fig. \[isofotas\] as isophote maps of our 20 galaxies.

To calculate the warp curves, we fitted Gaussians to the vertical profiles of the galaxies in each filter. We only considered data with a signal-to-noise 