  Star formation induced by a spiral shock wave, which in turn is generated by
a spiral density wave, produces an azimuthal age gradient across the spiral
arm, which has opposite signs on either side of the corotational resonance. An
analysis of the spatial separation between young star clusters and nearby HII
regions made it possible to determine the position of the corotation radius in
the studied galaxies. Fourier analysis of the gas velocity field in the same
galaxies independently confirmed the corotation radius estimates obtained by
the morphological method presented here.
