In previous articles (Dupraz & Combes, 1985, 1986a), we showed that shells form with different geometries around prolate and oblate galaxies. However, theory and observations suggest that some ellipticals could be tumbling bars (Miller & Smith 1980; Müllenhoff & Marenbach 1986). Here we simulate the accretion of a small galaxy by a tumbling bar; the tumble period T& is kept free. Let Tp be the typical period of motion of a particle in the potential of the elliptical galaxy. Then we find (Dupraz & Combes, 1986b):
a) When Tt > 3TP (Figure l a), shells form with the geometry of a static prolate potential, i.e., aligned with the major axis.
(Figure lc), the particles feel the time-averaged potential, which is oblate: the shells display the typical oblate geometry. But there is no confusion with a static oblate shell galaxy, because the tumbling bars must be seen edge -on for the shells to appear.
c) When Tb
(Figure lb), the outer shells form with the oblate geometry, the inner shells with the prolate geometry. In between, no shells form, because particles follow resonant (non-radial) motions.