Left panel: Li I λ6707.8 equivalent widths as a function of effective temperatures for our targets. Since each symbol results from the analysis of a unique spectrum (Table 3), any source observed at multiple epochs is marked several times. The arrow symbols indicate the upper limits on lithium equivalent width. The lines of different color and style mark the boundaries for three young clusters. Right panel: lithium abundances versus effective temperatures. We disregard the upper limits. In both panels, we display the PMS-like (big filled circles), Pleiades-like (filled squares), and older (filled diamonds) stars (see Sect. 3.4 for details). The four comoving TTSs towards the CO Cepheus void (Paper II) and the additional members (Paper III) are highlighted with asterisks and crosses, respectively. For comparison, we also overplot the locus of the sources belonging to two young (age = 12−30 Myr) associations listed by da Silva et al. (2009) with different small triangles.