Best fit to the optical and NIR photometry and SWS spectrum of TT Cyg, with T_eff = 3200 K, log g =  −0.4, C/O = 1.10. The top panel shows the full SED, while the middle and lower panel show details of the SWS spectrum. The C₃ feature near 5.4 μm is not well fitted in any of the Aringer et al. (2009) models and is excluded from the fit (as indictated by the line between 4.4 and 6.1 μm). This is likely related to the issue discussed in Jörgensen et al. (2000) about the parameter K_p(C₃) that expresses the equilibrium between atomic carbon, C₂ and C₃ and that is not well known.