  The majority of Wolf-Rayet (WR) stars represent the stripped cores of evolved
massive stars who lost most of their hydrogen envelope. In low metallicity
environments, such as the Small Magellanic Cloud (SMC), stellar winds are
weaker and binary interaction is expected to dominate WR-star formation.
However, the WR binary fraction appears to be ~40% at any metallicity. We use
the recently determined physical properties of the twelve known SMC WR stars to
explore their possible formation channels through comparisons with grids of SMC
models, simulated with the detailed stellar evolution code MESA. These include
models of rapidly rotating single stars, which experience (partial) chemically
homogeneous evolution (CHE). We find that CHE is not able to account for the
majority of the SMC WR stars. However, the apparently single WN star SMC AB12
and the double WR system SMC AB5 (HD 5980) appear consistent with this channel.
We also analyze core helium burning stellar models assuming constant hydrogen
gradients in their envelopes. We find a dichotomy in the envelope hydrogen
gradients required to explain the observed temperatures of the SMC WR stars.
Shallow gradients are found for the WR stars with O star companions, consistent
with binary models where mass transfer occurs early, which is in agreement with
their binary properties. On the other hand, much steeper hydrogen gradients are
inferred for the group of hot apparently single WR stars. Since the hydrogen
profiles in post main sequence models of massive stars become steeper with
time, we conclude that these stars have likely been stripped by a companion
during a phase of common envelope evolution. The companions, either main
sequence stars or compact objects, are expected to still be present. A
corresponding search might identify the first immediate double black hole
binary progenitor with masses as high as those detected in GW150914.
