The AU Microscopii planetary system is only 24 Myr old, and its geometry may provide clues about the early dynamical
history of planetary systems. Here, we present the first measurement of the Rossiter–McLaughlin effect for the warm sub Neptune AU Mic c, using two transits observed simultaneously with the European Southern Observatory’s (ESO’s) Very
Large Telescope (VLT)/Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO),
CHaracterising ExOPlanet Satellite (CHEOPS), and Next-Generation Transit Survey (NGTS). After correcting for flares and for
the magnetic activity of the host star, and accounting for transit-timing variations, we find the sky-projected spin–orbit angle of
planet c to be in the range λc = 67.8+31.7 −49.0 degrees (1σ). We examine the possibility that planet c is misaligned with respect to the
orbit of the inner planet b (λb = −2.96+10.44 −10.30), and the equatorial plane of the host star, and discuss scenarios that could explain
both this and the planet’s high density, including secular interactions with other bodies in the system or a giant impact. We note
that a significantly misaligned orbit for planet c is in some degree of tension with the dynamical stability of the system, and with
the fact that we see both planets in transit, though these arguments alone do not preclude such an orbit. Further observations
would be highly desirable to constrain the spin–orbit angle of planet c more precisely.