  About half of the S0 galaxies in the nearby Universe show signatures of
recent or ongoing star formation. Whether these S0 galaxies were rejuvenated by
the accretion of fresh gas is still controversial. We study minor mergers of a
gas-rich dwarf galaxy with an S0 galaxy, by means of N-body smoothed-particle
hydrodynamics simulations. We find that minor mergers trigger episodes of star
formation in the S0 galaxy, lasting for ~10 Gyr. One of the most important
fingerprints of the merger is the formation of a gas ring in the S0 galaxy. The
ring is reminiscent of the orbit of the satellite galaxy, and its lifetime
depends on the merger properties: polar and counter-rotating satellite galaxies
induce the formation of long-lived smooth gas rings.
