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