The isotope ratio , ^ { 85 } Rb/ ^ { 87 } 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 Å for the diffuse gas toward \rho Oph A yields a value of 1.21 \pm 0.30 ( 1- \sigma ) that differs significantly from the meteoritic value of 2.59 .
The Rb/K elemental abundance ratio for the cloud also is lower than that seen in meteorites .
Comparison of the ^ { 85 } Rb/K and ^ { 87 } Rb/K ratios with meteoritic values indicates that the interstellar ^ { 85 } Rb abundance in this direction is lower than the Solar System abundance .
We attribute the lower abundance to a reduced contribution from the r -process .
Interstellar abundances for Kr , Cd , and Sn are consistent with much less r -process synthesis for the solar neighborhood compared to the amount inferred for the Solar System .