  The evolution of helium stars with initial masses in the range 1.6 to 120
Msun is studied, including the effects of mass loss by winds. These stars are
assumed to form in binary systems when their expanding hydrogenic envelopes are
promptly lost just after helium ignition. Significant differences are found
with single star evolution, chiefly because the helium core loses mass during
helium burning rather than gaining it from hydrogen shell burning. Consequently
presupernova stars for a given initial mass function have considerably smaller
mass when they die and will be easier to explode. Even accounting for this
difference, the helium stars with mass loss develop more centrally condensed
cores that should explode more easily than their single-star counterparts. The
production of low mass black holes may be diminished. Helium stars with initial
masses below 3.2 Msun experience significant radius expansion after helium
depletion, reaching blue supergiant proportions. This could trigger additional
mass exchange or affect the light curve of the supernova. The most common black
hole masses produced in binaries is estimated to be about 9 Msun. A new maximum
mass for black holes derived from pulsational pair-instability supernovae is
derived - 46 Msun, and a new potential gap at 10 - 12 Msun is noted. Models
pertinent to SN 2014ft are presented and a library of presupernova models is
generated.
