  We analyze the stop-band due to crab cavities for horizontal tunes that are
either close to integers or close to half integers. The latter case is relevant
for today's electron/positron colliders. We compare this stop-band to that
created by dispersion in an accelerating cavity and show that a single typical
crab cavity creates larger stop-bands than a typical dispersion at an
accelerating cavity.
  We furthermore analyze whether it is beneficial to place the crab cavity at a
position where the dispersion and its slope vanish. We find that this choice is
worth while if the horizontal tune is close to a half integer, but not if it is
close to an integer. Furthermore we find that stop-bands can be avoided when
the horizontal tune is located at a favorable side of the integer or the half
integer.
  While we are here concerned with the installation of a single crab cavity in
a storage ring, we show that the stop-bands can be weakened, although not
eliminated, significantly when two crab cavities per ring are chosen suitably.
