  Dense stellar environments as hosts of ongoing star formation increase the
probability of gravitational encounters among stellar systems during the early
stages of evolution. Stellar interaction may occur through non-recurring,
hyperbolic or parabolic passages (a so-called 'fly-by'), through secular binary
evolution, or through binary capture. In all three scenarios, the strong
gravitational perturbation is expected to manifest itself in the disc
structures around the individual stars. Here, we present near-infrared
polarised light observations that were taken with the SPHERE/IRDIS instrument
of three known interacting twin-disc systems: AS 205, EM* SR 24, and FU
Orionis. The scattered light exposes spirals likely caused by the gravitational
interaction. On a larger scale, we observe connecting filaments between the
stars. We analyse their very complex polarised intensity and put particular
attention to the presence of multiple light sources in these systems. The local
angle of linear polarisation indicates the source whose light dominates the
scattering process from the bridging region between the two stars. Further, we
show that the polarised intensity from scattering with multiple relevant light
sources results from an incoherent summation of the individuals' contribution.
This can produce nulls of polarised intensity in an image, as potentially
observed in AS 205. We discuss the geometry and content of the systems by
comparing the polarised light observations with other data at similar
resolution, namely with ALMA continuum and gas emission. Collective
observational data can constrain the systems' geometry and stellar
trajectories, with the important potential to differentiate between dynamical
scenarios of stellar interaction.
