---
author:
- 'A. Guijarro, R. F. Peletier, E. Battaner, J. Jiménez-Vicente, R. de Grijs'
- 'E. Florido'
date: 'Received / Accepted'
title: 'Near-infrared and optical observations of galactic warps: A common, unexplained feature of most discs'
---

Introduction
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Most spiral galaxies, including our own, exhibit warped discs. This has long been known based on observations of both the extended neutral-gas (HI) component (Sancisi 1976; Bosma 1981; Briggs 1990; García-Ruiz et al. 2002b) and optical starlight (Sánchez-Saavedra et al. 1990; Florido et al. 1991; Reshetnikov & Combes 1998; Ann & Park 2006). The first detections of warped discs came from 21 cm line observations of our own Galaxy (Burke 1957; Kerr 1957; Burton 1988). Based on a sky survey covering the Northern Hemisphere, Sánchez-Saavedra et al. (1990) first reported the high frequency of optical warps in disc galaxies, which they later confirmed in Sánchez-Saavedra et al. (2003) based on an extended catalogue that also included the Southern Hemisphere. These studies took into account that warps can only be detected when their orientation (projection) is favourable (i.e., not in the line of sight). A noteworthy and important result, established in the later paper, is that they did not find any 