ratio greater than 3 ($\sigma$ = $\sqrt{\sigma^{2}_{\rm{sys}} + \sigma^{2}_{\rm{std}}}$, where $\sigma_{\rm{sys}}$ is the systematic error associated with the random background noise and $\sigma_{\rm{std}}$ the standard deviation), and with the FWHM of the peak smaller than 100 pixels. The results of our analysis are presented in Fig. \[alabeos\], where the warp curves are shown only for those values with error bars smaller than 0.5$\arcsec$. This estimated error bar can be computed by scaling the standard deviation ($1\sigma$ error) by the measured $\chi^2$ value.

Warp parameters {#warppar}
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It is convenient to use a geometric definition of a warp, e.g., for statistical studies. For an edge-on galaxy, the suitable coordinates correspond to the two directions contained in the plane of the sky: the direction defined by the major axis of the galaxy, $x$, and the direction of the rotation axis of the galaxy, $y$. The centre of the galaxy is the origin of coordinates. A warp curve is defined as the locus of points ($x_i, y_i$) tracing the distribution of the highest intensities, where $y_i$ is the position of the centre of the best-fitting Gaussian at a given $x_i$.

We derived the parameter $\omega$ (defined in Jiménez-Vicente et al. 1997), $$\omega=\frac{1}{L^3} \int_{-L/2}^{+L/2} 