Constraints in the mass-luminosity plane for secondary components for fixed q, T_eff(1), and absolute magnitudes, obtained by assuming that the primaries fill their Roche lobes (cp. Fig. 13); refer to Sect. 5.5.2 for details. Thick black solid lines show M–L loci for the indicated secondary temperatures (in kK) over a range in continuum brightness ratio, ℱ₂/ℱ₁ (values shown in the lower horizontal scales, marked at steps of 0.2). The secondary mass M is constant for given ℱ₂/ℱ₁(for fixed q values), as is the orbital inclination (upper horizontal scales, labelled in degrees). Thin solid and dashed curves show the zero-age and terminal-age main sequence loci for non-rotating single stars (from Brott et al. 2011a,b). Grey shaded areas indicate the zones for which 45° ≤ i ≤ 60°, L ≥ L(ZAMS), and ℱ₂/ ℱ₁ ≤ 1. Filled circles show the initial parameter estimates summarised in Table 8 (columns headed “M1”). Open circles represent equivalent results for V-band photometry.