  Yellow and red supergiants are evolved massive stars whose numbers and
locations on the HR diagram can provide a stringent test for models of massive
star evolution. Previous studies have found large discrepancies between the
relative number of yellow supergiants observed as a function of mass and those
predicted by evolutionary models, while a disagreement between the predicted
and observed locations of red supergiants on the HR diagram was only recently
resolved. Here we extend these studies by examining the yellow and red
supergiant populations of M33. Unfortunately, identifying these stars is
difficult as this portion of the color-magnitude diagram is heavily
contaminated by foreground dwarfs. We identify the red supergiants through a
combination of radial velocities and a two-color surface gravity discriminant
and, after re-characterizing the rotation curve of M33 with our newly selected
red supergiants, we identify the yellow supergiants through a combination of
radial velocities and the strength of the OI $\lambda$7774 triplet. We examine
~1300 spectra in total and identify 121 yellow supergiants (a sample which is
unbiased in luminosity above log(L/L\odot) ~ 4.8) and 189 red supergiants.
After placing these objects on the HR diagram, we find that the latest
generation of Geneva evolutionary tracks show excellent agreement with the
observed locations of our red and yellow supergiants, the observed relative
number of yellow supergiants with mass and the observed red supergiant upper
mass limit. These models therefore represent a drastic improvement over
previous generations.
