  The ALEPH Silicon Vertex Detector for LEP2 featured a laser survey system to
monitor its mechanical stability. The analysis of laser system data from 1997
to 2000 showed that VDET suffered a time-dependent displacement. It resulted to
be compatible with a deformation of the support structure that made the device
to slowly rotate during the data-taking. A maximal local displacement of ~20
microns was observed, corresponding to a rotation of ~10E-4 rad. The
implementation of a time-dependent correction on the alignment by using the
laser system data led to sizeable improvements on the ALEPH data quality.
