Hypernuclei containing a Ξ baryon are known. Empirical studies and theoretical models indicate that the Ξ –proton interaction is attractive, but weaker than the Λ–nucleon interaction. Like the Σ and other negatively charged particles, the Ξ can also form an exotic atom. When a Ξ is bound in an exotic atom or a hypernucleus, it quickly decays to a ΛΛ hypernucleus or to two Λ hypernuclei by exchanging a strange quark with a proton, which releases about 29 MeV of energy in free space: