  We present a new method for estimating the corotation radius in tightly wound
spiral galaxies, through analysis of the radial variation of the offset between
arms traced by the potential (P-arms) and those traced by dust (D-arms). We
have verified the predictions of semi-analytical theory through hydrodynamical
simulations and have examined the uniqueness of the galactic parameters that
can be deduced by this method. We find that if the range of angular offsets
measured at different radii in a galaxy is greater than around pi/4, it is
possible to locate the radius of corotation to within ~ 25%. We argue that the
relative location of the P- and D-arms provides more robust constraints on the
galactic parameters than can be inferred from regions of enhanced star
formation (SF-arms), since interpretation of the latter involves uncertainties
due to reddening and the assumed star formation law. We thus stress the
importance of K-band studies of spiral galaxies.
