Evolution of the fundamental-mode period and luminosity during the TP-AGB based on the evolutionary models of Marigo et al. (2013) computed with the COLIBRI code. Initial masses (in M_⊙) are labelled and the metallicity is Z = 0.006. The stages with C/O < 1 are shown in blue and with C/O > 1 in red. The observed points for the 2 OH/IR stars and 4 C-stars are indicated with the blue circles, and red diamonds, respectively.