# 7.2.1 Overview

The motion of an unresolved binary can be described with several models, depending on its orbital period: when it is much longer than the mission time interval, the trajectory is like a parabolic arc and it is sufficient to add a constant acceleration to the single-star model; this is what the so-called “constant acceleration” model does (referred to as Acceleration7). When slightly shorter periods are considered, the trajectory of the photocentre is still an arc, but it is necessary to include the time derivative of the acceleration. The model is then called the “variable acceleration model” (referred to as Acceleration9). When the period does not exceed the mission time interval, it is possible to calculate the orbit parameters by fitting an “orbital model”. Finally, the photocentre may also move between the two components as a result of photometric fluctuations in one of them. The double star then belongs to the category of variability-induced movers (VIM, Section 7.2.6). Among the different types of VIM models, only the VIM with fixed components (VIMF) have provided solutions that have been retained.

All these models were tried one after the other, until one produced a sufficiently convincing solution that it did not seem worth trying another. Otherwise, when even the VIMF model was not suitable, the single-star solution of the Gaia DR3 was retained. Subsequently, it became apparent that the acceptance of the solutions had not been sufficiently tight, and some solutions were rejected at a post-processing level. In this case too, the absence of a NSS solution means that the single-star solution of Gaia DR3 was retained, with no special indication of a potential non-single star nature in the catalogue beside the various ruwe or IPD values.