The observed composite spectrum, ϕ = 0.819, of the semidetached binary ET Cru (black) and its decomposition into the individual contributions of the primary (blue) and secondary (red) components in the 4000–5000 Å region. This disentangling, essential for isolating the spectral features of each star, provided the foundation for the subsequent determination of orbital dynamics, atmospheric parameters, and detailed chemical abundances.