The magnitude of three-phase fault currents at buses $A$ and $B$ of a power system are $10$ pu and $8$ pu, respectively. Neglect all resistances in the system and consider the pre-fault system to be unloaded. The pre-fault voltage at all buses in the system is $1.0$ pu. The voltage magnitude at bus $B$ during a three-phase fault at bus $A$ is $0.8$ pu. The voltage magnitude at bus $A$ during a three-phase fault at bus $B$, in pu, is __________.