Cepheids and spiral structure.

The Cepheid distributions in the galactic plane and in M31 are compared, and information available on the Cepheids in two other spiral galaxies, M33 and NGC2403, is considered. At 10-12 kpc galactocentric distance the Cepheid distributions in the galactic plane and in M31 differ markedly, perhaps because in the Galaxy, but not in M31, this distance is approximately the corotation radius. If the galactic Carina-Sagittarius arm is 2-3 kpc wide, it may be analogous to M31 arm S4, where Cepheids of all periods occur and which exhibits a transverse gradient in the age of the youngest member stars. In the Galaxy, M31, and probably Sb galaxies generally, Cepheid periods are longest not near the center but in annular zones of intensive star formation.