Cepheids and spiral structure

The great difference is found between the distributions of Cepheids in the planes of the Galaxy and M31 at the distance of 10-12 kpc from the centre which is probably due to the nearness of this distance to the corotation radius in the case of the Galaxy but not M31. It is noted that provided the width of the Carina-Saggittarius arm in the Galaxy is 2-3 kpc this arm may turn out to be an analogue to the S4 arm in M31, which reveal a concentration of the Cepheids of all periods and a transversal gradient of the youngest stellar ages. The periods of Cepheids in the Galaxy and M31 (and probably in general in other Sb galaxies) increase not toward the centre but toward the circular regions of the maximal star formation rate