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We report the first detection of C$_2$ $A^1\Pi_u$--$X^1\Sigma_g^+$ (0,0) andCN $A^2\Pi_u$--$X^2\Sigma^+$ (0,0) absorption bands in the interstellar medium.The detection was made using the near-infrared (0.91--1.35 $\mu$m)high-resolution ($R=20,000$ and 68,000) spectra of Cygnus OB2 No.\,12 collectedwith the WINERED spectrograph mounted on the 1.3 m Araki telescope. The$A$--$X$ (1,0) bands of C$_2$ and CN were detected simultaneously. Thesenear-infrared bands have larger oscillator strengths, compared with the$A$--$X$ (2,0) bands of C$_2$ and CN in the optical. In the spectrum of theC$_2$ (0,0) band with $R=68,000$, three velocity components in the line ofsight could be resolved and the lines were detected up to high rotationallevels ($J''\sim20$). By analyzing the rotational distribution of C$_2$, wecould estimate the kinetic temperature and gas density of the clouds with highaccuracy. Furthermore, we marginally detected weak lines of $^{12}$C$^{13}$Cfor the first time in the interstellar medium. Assuming that the rotationaldistribution and the oscillator strengths of the relevant transitions of$^{12}$C$_2$ and $^{12}$C$^{13}$C are the same, the carbon isotope ratio wasestimated to be $^{12}\text{C}/^{13}\text{C}=50$--100, which is consistent withthe ratio in the local interstellar medium. We also calculated the oscillatorstrength ratio of the C$_2$ (0,0) and (1,0) bands from the observed bandstrengths. Unfortunately, our result could not discern theoretical andexperimental results because of the uncertainties. High-resolution data toresolve the velocity components will be necessary for both bands in order toput stronger constraints on the oscillator strength ratios.

First detection of $A$--$X$ (0,0) bands of interstellar C$_2$ and CN