  Yellow straggler stars (YSSs) fall above the subgiant branch in optical
color-magnitude diagrams, between the blue stragglers and the red giants. YSSs
may represent a population of evolved blue stragglers, but none have the direct
and precise mass and radius measurements needed to determine their evolutionary
states and formation histories. Here we report the first asteroseismic mass and
radius measurements of such a star, the yellow straggler S1237 in the open
cluster M67. We apply asteroseismic scaling relations to a frequency analysis
of the Kepler K2 light curve and find a mass of 2.9 $\pm$ 0.2 M$_{\odot}$ and a
radius of 9.2 $\pm$ 0.2 R$_{\odot}$. This is more than twice the mass of the
main- sequence turnoff in M67, suggesting S1237 is indeed an evolved blue
straggler. S1237 is the primary in a spectroscopic binary. We update the binary
orbital solution and use spectral energy distribution (SED) fitting to
constrain the color-magnitude diagram (CMD) location of the secondary star. We
find that the secondary is likely an upper main-sequence star near the turnoff,
but a slightly hotter blue straggler companion is also possible. We then
compare the asteroseismic mass of the primary to its mass from CMD fitting,
finding the photometry implies a mass and radius more than 2$\sigma$ below the
asteroseismic measurement. Finally, we consider formation mechanisms for this
star and suggest that S1237 may have formed from dynamical encounters resulting
in stellar collisions or a binary merger.
