Wide binary stars are born from the same molecular cloud, at the same time, with the same initial composition. If lithium depletion depends only on stellar properties, both components should show the same lithium abundance at the same effective temperature.
A survey of 116 wide binary pairs confirms that temperature dominates lithium depletion, with rotation playing only a secondary role. The lithium-temperature relation shows the expected features: a dip between 6200 and 6600 K, a plateau between 6000 and 6200 K, and a steady decline for cooler stars. The pattern is consistent across binary pairs — same age, same composition, same result.
Except for one system. In the lithium dip region, one pair shows a 1.4 dex discrepancy — the primary star has roughly 25 times more lithium than its companion at nearly identical temperature. Both stars should have depleted their lithium to the same level. One of them didn't.
Two explanations survive: the lithium-rich star swallowed a planet or planetesimal disk, adding external lithium to its convective envelope; or a hidden third body in the system altered the binary's evolution through gravitational interactions, changing the internal mixing history of one component.
The anomaly is informative precisely because the control is so clean. In a field sample, a lithium-rich star is unremarkable — it could simply have started with more. In a coeval binary, the starting conditions are identical, so the excess must have been acquired. The binary provides the controlled experiment that field stars cannot.