The comparison between the surface chemistry of J053520.69 and the time variation of chemical composition of low-mass model stars. Left: time variation in the surface C/O ratio of a 1 M_⊙, metal-poor model star during the AGB phase, until post-AGB evolution. The red square and the red line indicate the measured value and the error bar from van Aarle et al. (2013). Right: time evolution of the surface carbon of two model stars of mass 1 M_⊙ (solid black line) and 1.1 M_⊙ (dotted green line), compared with results from van Aarle et al. (2013).