  We discuss the spin-orbit orientation of the Fomalhaut planetary system
composed of a central A4V star, a debris disk, and a recently discovered
planetary companion. We use spectrally resolved, near-IR long baseline
interferometry to obtain precise spectro-astrometric measurements across the
brG absorption line. The achieved astrometric accuracy of 3 nu-as and the
spectral resolution R=1500 from the AMBER/VLTI instrument allow us to spatially
and spectrally resolve the rotating photosphere. We find a position angle
PAstar=65deg pm 3deg for the stellar rotation axis, perfectly perpendicular
with the literature measurement for the disk position angle (PAdisk=156deg pm
0.3deg). This is the first time such test can be performed for a debris disk,
and in a non-eclipsing system. Additionally, our measurements suggest
unexpected backward-scattering properties for the circumstellar dust grains.
Our observations validate the standard scenario for star and planet formation,
in which the angular momentum of the planetary systems are expected to be
collinear with the stellar spins.
