Isotopic ratios at z=0.68 from molecular absorption lines toward B
  0218+357
  Isotopic ratios of heavy elements are a key signature of the nucleosynthesis
processes in stellar interiors. The contribution of successive generations of
stars to the metal enrichment of the Universe is imprinted on the evolution of
isotopic ratios over time. We investigate the isotopic ratios of carbon,
nitrogen, oxygen, and sulfur through millimeter molecular absorption lines
arising in the z=0.68 absorber toward the blazar B 0218+357. We find that these
ratios differ from those observed in the Galactic interstellar medium, but are
remarkably close to those in the only other source at intermediate redshift for
which isotopic ratios have been measured to date, the z=0.89 absorber in front
of PKS1830-211. The isotopic ratios in these two absorbers should reflect
enrichment mostly from massive stars, and they are indeed close to the values
observed toward local starburst galaxies. Our measurements set constraints on
nucleosynthesis and chemical evolution models.
