  The yellow supergiant content of nearby galaxies can provide a critical test
of stellar evolution theory, bridging the gap between the hot, massive stars
and the cool red supergiants. But, this region of the color-magnitude diagram
is dominated by foreground contamination, requiring membership to somehow be
determined. Fortunately, the large negative systemic velocity of M31, coupled
to its high rotation rate, provides the means for separating the contaminating
foreground dwarfs from the bona fide yellow supergiants within M31. Using the
MMT, we obtained spectra of about 2900 stars, selected using the color and
magnitude range to be yellow supergiants. Comparing the velocities to that of
M31's rotation curve, we identified 54 certain, and 66 probable yellow
supergiants from among the sea of foreground dwarfs. We find excellent
agreement between the location of yellow supergiants in the H-R diagram and
that predicted by the latest Geneva evolutionary tracks which include rotation.
However, the relative number of yellow supergiants seen as a function of mass
varies from that predicted by the models by a factor of more than 10, in the
sense that more high mass yellow supergiants are predicted than are actually
observed. Comparing the total number of yellow supergiants with masses above
20Mo with the estimated number of unevolved O stars indicates that the duration
of the yellow supergiant phase is about 3000 years. This is consistent with
what the 12Mo and 15Mo evolutionary tracks predict, but disagrees with the
20,000-80,000 year time scales predicted by the models for higher masses.
