The isotope ratio Rb/Rb places constraints on models of the nucleosynthesis of heavy elements, but there is no precise determination of the ratio for material beyond the solar system. We report the first measurement of the interstellar Rb isotope ratio. Our measurement of the Rb i line at 7800 for the diffuse gas toward r Oph A yields Å a value of (1 j) that differs significantly from the meteo...
