Over the past 7 years, (CH+)-C-12/(CH+)-C-13 observations performed by different groups of authors at various observatories toward the zeta Oph cloud have yielded interstellar C-12/C-13 isotope ratios varying at the factor of 2 level. In an effort to determine the cause of this discrepancy, we have (1) reanalyzed the Lick Observatory Reticon data presented in Hawkins, Jura, & Meyer, (2) obtained a new set of Lick Observatory observations toward zeta Oph using a CCD detector, and (3) carried out high-resolution, high signal-to-noise ratio observations toward zeta Oph and xi Per using the KPNO coude feed echelle spectrograph and a CCD detector. The reanalysis of the Lick Observatory Reticon data, performed with different software and using a different way of placing the continuum, has yielded (CH+)-C-12/(CH+)-C-13 ratios that are completely consistent with the original analysis presented in the Hawkins, Jura, & Meyer paper. The Lick Observatory CCD data toward zeta Oph suffer from a large amount of telluric contamination, and the C-12/C-13 results are quite uncertain, ranging from 40 to 60, owing to the error in continuum placement. The KPNO CCD data are of highest quality in terms of resolution, signal-to-noise ratio, and absence of telluric lines. These KPNO data cover the CH+ 4232 and 3957 angstrom regions for zeta Oph and xi Per. In the case of zeta Oph, the KPNO data also yielded a measurement of the (CN)-C-12/(CN)-C-13 ratio at 3874 angstrom. The KPNO results toward zeta Oph are (CH+)-C-12/(CH+)-C-13(0, 0) = 63 +/- 8, (CH+)-C-12/(CH+)-C-13 (1, 0) = 67 +/- 19, and (CN)-C-12/(CN)-C-13 = 100(-33)+88). Toward xi Per, our KPNO measurements yield (CH+)-C-12/(CH+)-C-13 49 +/- 15 from the 4232 angstrom band, and a lower limit of 45 from the 3957 angstrom observations. Our KPNO CCD (CH+)-C-12/(CH+)-C-13 and (CN)-C-12/(CN)-C13 zeta Oph results presented here are consistent with recent determinations by Crane, Hegyi & Lambert, Stahl & Wilson, and Kaiser & Wright. By contrast, the work by Hawkins, Jura, & Meyer and Crane & Hegyi deviates beyond the 1 sigma errors from the above results. Toward xi Per, our KPNO CCD (CH+)C-12/(CH+)-C-13 results agree with the Hawkins & Jura measurements. The CH+ molecule provides the best probe of the interstellar C-12/C-13 ratio in the gas because it is not believed to suffer from isotope-selective effects such as chemical fractionation. A comparison of the solar system C-12/C-13 ratio of 89 to the (CH+)-C-12/(CH+)-C-13 KPNO values of 63 and 49 which we measure in the interstellar medium toward zeta Oph and xi Per, respectively, implies that the Galaxy has undergone a significant amount of chemical evolution since the formation of the solar system.
