Analysis of association #135, color–magnitude (top) and color–color (bottom) diagrams. Clockwise starting from top-left: a) B−V vs. V, b) Q vs. V_corr, c) U−B vs. Q and d) B−V vs. U−B. The catalog of IC 1613 targets with good photometric quality is plotted with green dots. The likely OB stars of the association are the blue circles, and the non-OB members are represented in red. The error bars represent photometric errors. Diamonds in the Q vs. V_corr diagram represent the OB members, their V-magnitude corrected for extinction (see Sect. 2.1). Note the larger error bars in this diagram, due to error propagation. In all panels except for the B−V vs. U−B  diagram, colored lines represent the 0.2 Z_⊙ isochrones of Lejeune & Schaerer (2001), whose log age is color–coded as indicated in panel a). In the B−V vs. U−B diagram, the dashed lines represent the reddening vector for O3–O9 stars. The other lines are the calibration of color with spectral type of Allen (2000) for O5-M5 main sequence stars (green), G5-M5 giants (blue), and O3-M5 supergiants (pink).