Although interarm features in the Population I disk of spiral galaxies frequently give an impression of a three-fold symmetry (Patsis et al., 1997), true three-armed spiral structures in the old stellar disk are seldomly seen. Such systems are of special interest since they display unique conditions which favor the growth of m=3 modes. The face-on spiral NGC 7137 shows a clear three-armed pattern on K-band images and was used as a prototype for the potential of these systems.

A K-band map of NGC 7137 was decomposed in axisymmetric components and a synthetic rotation curve was generated [see (Grosbøl and Patsis, 1998)]. The maximum rotational velocity was taken to be ≈150 km sec−1 corresponding to the mean value given by (Rubin et al., 1982) for this type of galaxy.