  We present optical and near-infrared observations of SN~Ib~2019ehk. We show
that it evolved to a Ca-rich transient according to its spectral properties and
evolution in late phases. It, however, shows a few distinguishable properties
from the canonical Ca-rich transients: a short-duration first peak in the light
curve, high peak luminosity, and association with a star-forming environment.
Indeed, some of these features are shared with iPTF14gqr and iPTF16hgs, which
are candidates for a special class of core-collapse SNe (CCSNe): the so-called
ultra-stripped envelope SNe, i.e., a relatively low-mass He (or C+O) star
explosion in a binary as a precursor of double neutron star binaries. The
estimated ejecta mass ($0.43 M_\odot$) and explosion energy ($1.7 \times
10^{50} $~erg) are consistent with this scenario. The analysis of the first
peak suggests existence of dense circumstellar material in the vicinity of the
progenitor, implying a CCSN origin. Based on these analyses, we suggest SN
2019ehk is another candidate for an ultra-stripped envelope SN. These
ultra-stripped envelope SN candidates seem to form a subpopulation among
Ca-rich transients, associated with young population. We propose that the key
to distinguishing this population is the early first peak in their light
curves.
