  Warps in the outer gaseous disks of galaxies are a ubiquitous phenomenon, but
it is unclear what generates them. One theory is that warps are generated
internally through spontaneous bending instabilities. Other theories suggest
that they result from the interaction of the outer disk with accreting
extragalactic material. In this case, we expect to find cases where the
circular velocity of the warp gas is poorly correlated with the rotational
velocity of the galaxy disk at the same radius. Optical spectroscopy presents
itself as an interesting alternative to 21-cm observations for testing this
prediction, because (i) separating the kinematics of the warp from those of the
disk requires a spatial resolution that is higher than what is achieved at 21
cm at low HI column density; (ii) optical spectroscopy also provides important
information on star formation rates, gas excitation, and chemical abundances,
which provide clues to the origin of the gas in warps. We present here
preliminary results of a study of the kinematics of gas in the outer-disk warps
of seven edge-on galaxies, using multi-hour VLT/FORS2 spectroscopy.
