Mass of a putative third companion as a function of its orbital separation relative to the binary, required to reproduce the observed apsidal precession of DQ Tau. The solid blue curve shows the corresponding mass estimate, the shaded grey area indicates the uncertainty range, and the vertical dashed magenta line marks the predicted cavity radius(~3a_bin), based on DQ Tau orbital properties and assuming a circular coplanar disc (Ragusa et al. 2025).