Search for ¹³C¹²C (2-0) Phillips transitions. Top panel: spectral segments of HD 63804, HD 147889, and HD 169454 around the target wavelength region, and simulated data (red) for ¹³C¹²C with T_exc = 55 K. From left to right, the four transitions are Q(1), Q(2), Q(3), and Q(4). Unfortunately, we cannot identify these features in any individual sightline. Bottom panel: stacked (average) data of the three sightlines. We have a 3σ detection of the Q(2) and Q(3) transitions, and a marginal detection of the Q(4) transition. The estimated column density of ¹³C¹²C is 2.30 ± 0.31 × 10¹²cm⁻² in the stacked spectrum, corresponding to a ¹²C/¹³C ratio of 79±8. The dashed blue lines around the spectra represent 1σ uncertainties.