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The fate of stars in the zero-age main-sequence (ZAMS) range $\approx 8-12$Msun is unclear. They could evolve to form white dwarfs or explode aselectron-capture supernovae (SNe) or iron core-collapse SNe (CCSNe). Eventhough the initial mass function indicates that this mass range should accountfor over 40% of all CCSNe progenitors, few have been observationally confirmed,likely owing to the faintness and rapid evolution of these transients. In thispaper, we present a sample of nine Ca-rich/O-poor Type IIb SNe detected by theZwicky Transient Facility with progenitors likely in this mass range. Weperform a holistic analysis of the spectroscopic and photometric properties ofthe sample. These sources have a flux ratio of [Ca II] $\lambda \lambda$7291,7324 to [O I] $\lambda \lambda$6300, 6364 of $\gtrsim$ 2 in their nebularspectra. Comparing the measured [O I] luminosity ($\lesssim 10^{39}\mathrm{erg\ s^{-1}}$) and derived oxygen mass ($\lesssim 0.1$ Msun) withtheoretical models, we infer that the progenitor ZAMS mass for these explosionsis less than 12 Msun. These correspond to He-stars with core masses less thanaround 3 Msun. We find that the ejecta properties (Mej $\lesssim 1$ Msun) arealso consistent with those expected for such low mass He-stars. The low ejectamass of these sources indicates a class of strongly-stripped SNe that is atransition between the regular stripped-envelope SNe and ultra-stripped SNe.The progenitor could be stripped by a main sequence companion and result in theformation of a neutron star $-$ main sequence binary. Such binaries have beensuggested to be progenitors of neutron star $-$ white dwarf systems that couldmerge within a Hubble time, and be detectable with LISA.

Probing the low-mass end of core-collapse supernovae using a sample of  strongly-stripped Calcium-rich Type IIb Supernovae from the Zwicky Transient  Facility