Same as Fig. 6; stars displaying noticeable LaO bands are plotted as filled triangles. Most of them are pure S stars or SC stars. The location of M (C/O ~0.5), S (0.5< C/O <1) and SC stars (C/O ~1) is indicated, as well as some typical effective temperatures. The arrow is an approximative indication of the loop described in this diagram by the stars as they evolve on the TPAGB, as explained in Sect. 3.4.