Numerical orbit models (Patsis et al. 1991, and references therein; Patsis et al. 1994) seem to indicate that, because of the stochasticity of stellar orbits near corotation, spiral arms end at corotation or at the 4:1 inner Lindblad resonance (ILR), depending on the strength of the perturbation. These authors also claim that spiral structure extending beyond corotation as far as the outer Lindblad resonance (OLR) would be typical of barred spirals with their bars ending at corotation; otherwise a conspicuous gap in the spiral arms would appear. However, under conditions extracted from observations, N-body numerical simulations produce persistent spiral arms (Thomasson et al. 1990) which end at the OLR. According to the spiral density wave theory, although corotation is a singular region, short trailing waves can exist beyond corotation out to the OLR, while inside corotation, both short and long trailing and leading waves can exist (see, for example, Lin & Lau 1979, and references therein). On the other hand, several observational features can be readily understood if it is assumed that they mark the corotation region, thus implying that spiral arms end at the OLR, including spurs, gaps and interarm star formation (Elmegreen et al. 1989), breaks, bifurcations, changes of pitch angle, lower star formation in the arms compared with interarm star formation (Cepa & Beckman 1990a,b) and changes of arm skewness (Paper III). These features are usually present one at a time, and the gaps predicted by Patsis et al. (1991) are not generally present. Anyway, numerical simulations, analytical solutions and observational evidence coincide in that corotation is a singular region where the behaviour of density waves is not easy to predict. In this paper (the last of a series of four) we use Fourier transforms to complete the analysis of the sample of S(b-c) and SB(b-c) galaxies described in del Río & Cepa (1998a, hereafter Paper II), to analyse arm behaviour in the neighbourhood of the resonances and corotation determined in Paper III, together with the relative importance of the different arm components in these zones. Also, we compare the results with those obtained using the method of EEM.
In Sect. 2 we give a brief description of the data and the mathematical methods. Section 3 presents the analysis of spiral structure. Section 4 concerns to detailed analysis of each galaxy, and finally Sect. 5 shows the conclusions.
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