  Models of stellar structure and evolution can be constrained using accurate
measurements of the parameters of eclipsing binary members of open clusters.
Multiple binary stars provide the means to tighten the constraints and, in
turn, to improve the precision and accuracy of the age estimate of the host
cluster. In the previous two papers of this series, we have demonstrated the
use of measurements of multiple eclipsing binaries in the old open cluster
NGC6791 to set tighter constraints on the properties of stellar models than was
previously possible, thereby improving both the accuracy and precision of the
cluster age. We identify and measure the properties of a non-eclipsing cluster
member, V56, in NGC\,6791 and demonstrate how this provides additional model
constraints that support and strengthen our previous findings. We analyse
multi-epoch spectra of V56 from FLAMES in conjunction with the existing
photometry and measurements of eclipsing binaries in NGC6971. The parameters of
the V56 components are found to be $M_{\rm p}=1.103\pm 0.008 M_{\odot}$ and
$M_{\rm s}=0.974\pm 0.007 M_{\odot}$, $R_{\rm p}=1.764\pm0.099 R_{\odot}$ and
$R_{\rm s}=1.045\pm0.057 R_{\odot}$, $T_{\rm eff,p}=5447\pm125$ K and $T_{\rm
eff,s}=5552\pm125$ K, and surface [Fe/H]=$+0.29\pm0.06$ assuming that they have
the same abundance. The derived properties strengthen our previous best
estimate of the cluster age of $8.3\pm0.3$ Gyr and the mass of stars on the
lower red giant branch (RGB), which is $M_{\rm RGB} = 1.15\pm0.02M_{\odot}$ for
NGC6791. These numbers therefore continue to serve as verification points for
other methods of age and mass measures, such as asteroseismology.
