  The progenitors of many Type II supernovae have been observationally
identified but the search for Type Ibc supernova (SN Ibc) progenitors has thus
far been unsuccessful, despite the expectation that they are luminous
Wolf-Rayet (WR) stars. We investigate how the evolution of massive helium stars
affects their visual appearances, and discuss the implications for the
detectability of SN Ibc progenitors. Massive WR stars that rapidly lose their
helium envelopes through stellar-wind mass-loss end their lives when their
effective temperatures -- related to their hydrostatic surfaces -- exceed about
150kK.At their pre-supernova stage, their surface properties resemble those of
hot Galactic WR stars of WO sub-type. These are visually faint with narrow-band
visual magnitudes Mv = -1.5 ~ -2.5, despite their high bolometric luminosities
(log L/Lsun = 5.6 ~ 5.7), compared to the bulk of Galactic WR stars (Mv < -4).
In contrast, relatively low-mass helium stars that retain a thick helium
envelope appear fairly bright in optical bands, depending on the final masses
and the history of the envelope expansion during the late evolutionary stages.
We conclude that SNe Ibc observations have so far not provided strong
constraints on progenitor bolometric luminosities and masses, even with the
deepest searches. We also argue that Ic progenitors are more challenging to
identify than Ib progenitors in any optical images.
