A unit for forming absorbent sanitary articles, comprising: - a drum rotatable about its axis of rotation and having a circumferential peripheral surface movable along a feed direction; - first feeding means for feeding a succession of pairs of panels of material facing the circumferential peripheral surface and configured to position each pair in such a way that the respective two panels are facing each other and transversely aligned with the feed direction; - second feeding means for feeding a continuous web of material suitable for defining at least part of the main body of an absorbent sanitary article, and configured to position the web on the circumferential peripheral surface of the drum in such a way that each panel projects partly sideways from the web; - an ultrasonic sealing station located along the circumferential peripheral surface of the drum, in a zone located operatively downstream of the second feeding means, and configured to attach each panel of the pair to the web at at least one plurality of sealing points arranged in succession along the feed direction; the ultrasonic sealing station comprises at least one first sonotrode and at least one second sonotrode facing the circumferential peripheral surface of the drum and spaced from each other transversely to the feed direction in such a way that each acts on a respective panel of each pair; characterized in that the first sonotrode and the second sonotrode are movable towards and away from each other in order to adapt to the size of the absorbent article and in that the drum has, along its circumferential peripheral surface, a plurality of suction cavities arranged in succession along an at least partly circumferential direction and a plurality of stops, or abutments, defining respective anvils for the sonotrodes and distributed both along the at least partly circumferential direction and along an axial direction, with reference to the axis of rotation of the drum. - a drum rotatable about its axis of rotation and having a circumferential peripheral surface movable along a feed direction; - first feeding means for feeding a succession of pairs of panels of material facing the circumferential peripheral surface and configured to position each pair in such a way that the respective two panels are facing each other and transversely aligned with the feed direction; - second feeding means for feeding a continuous web of material suitable for defining at least part of the main body of an absorbent sanitary article, and configured to position the web on the circumferential peripheral surface of the drum in such a way that each panel projects partly sideways from the web; - an ultrasonic sealing station located along the circumferential peripheral surface of the drum, in a zone located operatively downstream of the second feeding means, and configured to attach each panel of the pair to the web at at least one plurality of sealing points arranged in succession along the feed direction; the ultrasonic sealing station comprises at least one first sonotrode and at least one second sonotrode facing the circumferential peripheral surface of the drum and spaced from each other transversely to the feed direction in such a way that each acts on a respective panel of each pair;