  The structural parameters of a magnitude-limited sample of 148 unbarred S0-Sb
galaxies were derived using the GASP2D algorithm and then analyzed to derive
the intrinsic shape of their bulges. We developed a new method to derive the
intrinsic shape of bulges based on photometric data and on the geometrical
relationships between the apparent and intrinsic shapes of bulges and disks.
The method is conceived as completely independent of the studied class of
objects, and it can be applied whenever a triaxial ellipsoid embedded in an
axisymmetric component is considered. We found that the intrinsic shape is well
constrained for a subsample of 115 bulges with favorable viewing angles. A
large fraction of them is characterized by an elliptical section (B/A<0.9).
This fraction is 33%, 55%, and 43% if using their maximum, mean, or median
equatorial ellipticity, respectively. Most of them are flattened along their
polar axis. The distribution of triaxiality is strongly bimodal. This
bimodality is driven by bulges with Sersic index n>2, or equivalently, by the
bulges of galaxies with a bulge-to-total ratio B/T>0.3. Bulges with n<2 and
with B/T<0.3 follow a similar distribution, which is different from that of
bulges with n>2 and with B/T>0.3. In particular, bulges with n<2 and with
B/T<0.3 show a larger fraction of oblate axisymmetric (or nearly axisymmetric)
bulges, a smaller fraction of triaxial bulges, and fewer prolate axisymmetric
(or nearly axisymmetric) bulges with respect to bulges with n>2 and with
B/T>0.3, respectively. According to the numerical simulations of bulge
formation, bulges with n<2, which show a high fraction of oblate axisymmetric
(or nearly axisymmetric) shapes and have B/T<0.3, could be the result of
dissipational minor mergers. Both major dissipational and dissipationless
mergers seem to be required to explain the variety of shapes found for bulges
with n>2 and B/T>0.3.
