  When nuclear fuel in the core of a massive star with a zero-age main-sequence
mass $M_{\rm ZAMS} \gtrsim 8M_\odot$ is exhausted, the central part of the iron
or magnesium core collapses and forms a neutron star or a black hole. At the
same time, the material above the collapsing core is rapidly ejected, leading
to a stripped-envelope supernova (SESN) explosion if the outer hydrogen
envelope of the star was removed before its explosion. The envelope is
presumably stripped either via strong stellar winds or due to mass transfer to
a companion star in a close binary orbit. It is not clear which process is
dominant, and whether different mechanisms are at work for different classes of
SESNe; type IIb, Ib, and Ic SNe in order of increasing degree of envelope
stripping. In this work, a new analysis of late-time nebular spectra of SESNe
is presented, which is more sensitive to differences in the core structure than
early-phase spectral analysis. The results show that the progenitors of SNe IIb
and Ib are indistinguishable except for the residual amount of hydrogen
envelope while the progenitors of SNe Ic are not only deficient in hydrogen and
helium, but are also distinctly more massive than SNe IIb and Ib. These
findings strongly suggest that more than one mechanism is responsible for the
removal of the outer hydrogen envelope and the deeper helium layer, with the
former most likely due to binary interaction, and the latter involving a
mass-dependent process such as strong stellar winds or episodic pre-explosion
mass ejection.
