The spiral structure of M31: a morphological approach.

The problem of the spiral structure of M31 has been reconsidered, based on a new list of 981 H II regions and the OB associations of van den Bergh. The distribution of these arm tracers, as well as arguments concerning the H I density and an asymmetry in the rotation curve leads to the conclusion that the most probable solution for the unusual structure of M31 is a one-armed leading pattern (Kalnajs' solution), forced by the close companion M32, and that the classical two-armed trailing spiral is far less satisfactory.