  This paper presents a study of beam alignment in accelerating cavities based
on beam-excited higher order modes (HOM). Among the transverse HOMs, dipole
modes have the dominant contribution, and may have an adverse effect on the
beam quality. Since they depend linearly on the transverse beam offset, their
effect can be reduced by aligning the beam in the cavity. The study has been
made on the four third harmonic superconducting cavities installed in the ACC39
module at FLASH. Modes with strong beam coupling were monitored while moving
the beam and searching for the minimum excited HOM power.
