  We determine the properties of the binary star V106 in the old open cluster
NGC6791. We identify the system to be a blue straggler cluster member by using
a combination of ground-based and Kepler photometry and multi-epoch
spectroscopy. The properties of the primary component are found to be $M_{\rm
p}\sim1.67 \rm M_{\odot}$, more massive than the cluster turn-off, with $R_{\rm
p}\sim1.91 \rm R_{\odot}$ and $T_{\rm eff}=7110\pm100$ K. The secondary
component is highly oversized and overluminous for its low mass with $M_{\rm
s}\sim0.182 \rm M_{\odot}$, $R_{\rm s}\sim0.864 \rm R_{\odot}$ and $T_{\rm
eff}=6875\pm200$ K. We identify this secondary star as a bloated (proto)
extremely low-mass helium white dwarf. These properties of V106 suggest that it
represents a typical Algol-paradox system and that it evolved through a
mass-transfer phase which provides insight into its past evolution. We present
a detailed binary stellar evolution model for the formation of V106 using the
MESA code and find that the mass-transfer phase only ceased about 40 Myr ago.
Due to the short orbital period (P=1.4463 d) another mass-transfer phase is
unavoidable once the current primary star evolves towards the red giant phase.
We argue that V106 will evolve through a common-envelope phase within the next
100 Myr and merge to become a single over-massive giant. The high mass will
make it appear young for its true age, which is revealed by the cluster
properties. Therefore, V106 is potentially a prototype progenitor of old field
giants masquerading as young.
