Detection of a composite flux in Gaia photometry can serve as an indication of a photometrically unresolved binarity and can allow us to contribute to the parameterization of the components of binary systems. A main goal of the present study is to develop a method of automatic photometric detection, based on theoretical stellar spectral energy distributions and general understanding of binary evolution.

The following procedure is applied to achieve the declared goal. One can compose possible pairs of components, based on evolution concept, and using theoretical binary evolutionary tracks. This can be done for all stages of binary evolution, excluding perhaps the most marginal and rapid stages. Theoretical spectrophotometric spectral energy distributions and response functions of Gaia photometric bands allows us to compute colour-indices of such pairs, when they are unresolved, as well as of single stars. Usage of an interstellar extinction law gives us theoretical colour-indices of reddened objects, both single stars and unresolved binaries. When plotted on a multidimensional Gaia colour space, they allow us to indicate areas, where unresolved binaries can be easily separated from single stars. The procedure also indicates Gaia colour indices, suitable to distinguish those pairs from single stars.

As an output of the procedure, a Gaia list of detected photometrically unresolved binaries with physical parameters can be compiled.