  Fossil groups are considered the end product of natural galaxy group
evolution in which group members sink towards the centre of the gravitational
potential due to dynamical friction, merging into a single, massive, and X-ray
bright elliptical. Since gravitational lensing depends on the mass of a
foreground object, its mass concentration, and distance to the observer, we can
expect lensing effects of such fossil groups to be particularly strong. This
paper explores the exceptional system $\mathrm{J}143454.4+522850$. We combine
gravitational lensing with stellar population-synthesis to separate the total
mass of the lens into stars and dark matter. The enclosed mass profiles are
contrasted with state-of-the-art galaxy formation simulations, to conclude that
SW05 is likely a fossil group with a high stellar to dark matter mass fraction
$0.027\pm0.003$ with respect to expectations from abundance matching
$0.012\pm0.004$, indicative of a more efficient conversion of gas into stars in
fossil groups.
