  We investigate the origin of the type III (up-bending) discs based on a
sample of 1808 galaxies from SDSS and a sub-sample of 286 galaxies with HI data
from ALFALFA. We examine how the type III fraction f3, the fraction of disc
galaxies which host up-bending disc breaks, depends on other galactic
properties. We confirm that f3 strongly depends on the stellar concentration of
galaxies. We find that HI-rich galaxies with low spins tend to have
significantly more type III disc breaks than control galaxies, which are
matched in concentration and stellar mass. This trend is independent of the
existence of strong bars or environment of the galaxies. This result is broadly
consistent with predictions from theoretical simulations, and indicates in-situ
star formation fueled by gas accretion to be an important mechanism that builds
the outer discs of type III galaxies.
