The wide angle X-ray scattering pattern of mats of filtration of single crystals of poly(ethylene sebacate) and poly(ethylene suberate) presents subsidiary (non-Bragg) diffraction spots. The occurrence of such extra maxima is related to the extremely low number of repeat units in the crystalline part of the lamellae. The experimental intensity along the c* direction has been compared with structure factor determinations and WAXS simulations to determine the nature of the fold in single crystals of poly(ethylene sebacate). It has been found that the folds are made of the octamethylene groups contained in the sebacate parts of the macromolecules. The crystalline length l(c) obtained from the simulations is in good agreement with the experimental value of the long period when the description of the fold and of the interlamellar gap are taken into account.