  In some galaxies, the central velocity dispersion, sigma, is depressed with
respect to the surroundings. This sigma-drop phenomenon may have different
physical origins, bearing information about the internal dynamics of the host
galaxy. In this article, we stress the importance also of observational
artifacts due to the sigma-metallicity degeneracy: when a spectrum of a
population is compared with a template of miss-matched metallicity, the
velocity dispersion may be wrongly estimated. A sigma-drop may appear in place
of a metallicity peak. The discussion is illustrated using VLT/FORS spectra of
diffuse elliptical galaxies. Some of the sigma-drop galaxies reported in the
literature may be analysis artifacts.
