  Understanding how bodies interact with each other and with disk material
holds the key to understanding the architecture of stellar systems and of
planetary systems. While the interactions between point sources can be
described by simple gravity, interactions with disk material require further
knowledge about the gas viscosity and dust microphysics that needs to be
included when simulating disk-body interactions. Pre-main-sequence multiple
systems provide us with a unique laboratory to calibrate fundamental parameters
such as the viscosity and to test theories of hydrodynamic processes that might
shape protoplanetary disk structure and affect the planet populations forming
from these disks. In this article I briefly review our knowledge about a
particularly intriguing T Tauri triple star system, GW Orionis, that has the
potential to serve as a rosetta stone for hydrodynamic studies. The
3-dimensional orbits and masses of the stars in GW Orionis have been
constrained by long-term interferometric and radial velocity monitoring. Also,
the 3-dimensional geometry of the strongly distorted disk has been tightly
contrained based on high-angular resolution thermal dust emission and
scattered-light imaging. The disk-tearing effect that we might witness in GW
Ori in action constitutes an important new mechanism for moving disk material
onto highly oblique or retrograde orbits, even at very wide separations from
the star. At the same time, the observed torn ring seems sufficiently massive,
and might be sufficiently stable, for planet formation to occur, potentially
giving rise to an yet-undiscovered population of circum-multiple planets on
highly oblique, long-period orbits.

Solar and Stellar Astrophysics