  Massive stars (~8-25Msun) stripped of their hydrogen-rich envelopes via
binary interaction are thought to be the main progenitors for merging neutron
stars and stripped-envelope supernovae. We recently presented the discovery of
the first set of such stripped stars in a companion paper. Here, we fit the
spectra of ten stars with new atmosphere models in order to constrain their
stellar properties precisely. We find that the stellar properties align well
with the theoretical expectations from binary evolution models for helium-core
burning envelope-stripped stars. The fits confirm that the stars have high
effective temperatures (Teff~50-100kK), high surface gravities (log g ~5), and
hydrogen-poor/helium-rich surfaces (X(H, surf)~0-0.4) while showing for the
first time a range of bolometric luminosities (10^3-10^5 Lsun), small radii
(~0.5-1Rsun), and low Eddington factors (Gamma_e~0.006-0.4). Using these
properties, we derive intermediate current masses (~1-8Msun), which suggest
that their progenitors were massive stars (~5-25Msun) and that a subset will
reach core-collapse, leaving behind neutron stars or black holes. Using the
model fits, we also estimate the emission rates of ionizing photons for these
stars, which agree well with previous model expectations. Further, by computing
models for a range of mass-loss rates, we find that the stellar winds are
weaker than predicted by any existing scheme (Mdot(wind)<~ 1e-9 Msun/yr). The
properties of this first sample of intermediate mass helium stars suggest they
both contain progenitors of type Ib and IIb supernovae, and provide important
benchmarks for binary evolution and population synthesis models.
