Recent studies of old stars in the Galactic thick disc suggest it contains a population of “young” giant stars. These are chemically and kinematically members of the thick disc, but their masses -- measured by asteroseismology -- are up to a factor of two greater than expected given the age of the thick disc. Galactic astronomers have suggested they come from a population of stars that have migrated from the bar of the Galaxy. We show that they are binary stars which have undergone mass transfer, a process similar to that which forms blue stragglers. We populate the [C/N] vs mass parameter space with binary-star models which match the observations far better than we could have expected. We also predict binary fractions and other properties of these “young” stars, and hence show that these properties can potentially be used to pin down uncertain physics in binary systems, such as the efficiency of common envelope ejection.