  The origin of S0 galaxies is discussed in the framework of early mergers in a
Cold Dark Matter cosmology, and in a scenario where S0s are assumed to be
former spirals stripped of gas. From an analysis of 127 early-type disk
galaxies (S0-Sa), we find a clear correlation between the scale parameters of
the bulge (r_eff) and the disk (h_R), a correlation which is difficult to
explain if these galaxies were formed in mergers of disk galaxies. However, the
stripping hypothesis, including quiescent star formation, is not sufficient to
explain the origin of S0s either, because it is not compatible with our finding
that S0s have a significantly smaller fraction of bars (46$\pm$6 %) than their
assumed progenitors, S0/a galaxies (93$\pm$5 %) or spirals (64-69 %). Our
conclusion is that even if a large majority of S0s were descendants of spiral
galaxies, bars and ovals must play an important role in their evolution. The
smaller fraction particularly of strong bars in S0 galaxies is compensated by a
larger fraction of ovals/lenses (97$\pm$2 % compared to 82-83 % in spirals),
many of which might be weakened bars. We also found massive disk-like bulges in
nine of the S0 galaxies, bulges which might have formed at an early gas-rich
stage of galaxy evolution.
