  AU Mic is a young planetary system with a resolved debris disc showing signs
of planet formation and two transiting warm Neptunes near mean-motion
resonances. Here we analyse three transits of AU Mic b observed with the
CHaracterising ExOPlanet Satellite (CHEOPS), supplemented with sector 1 and 27
Transiting Exoplanet Survey Satellite (TESS) photometry, and the All-Sky
Automated Survey (ASAS) from the ground. The refined orbital period of AU Mic b
is 8.462995 \pm 0.000003 d, whereas the stellar rotational period is
P_{rot}=4.8367 \pm 0.0006 d. The two periods indicate a 7:4 spin--orbit
commensurability at a precision of 0.1%. Therefore, all transits are observed
in front of one of the four possible stellar central longitudes. This is
strongly supported by the observation that the same complex star-spot pattern
is seen in the second and third CHEOPS visits that were separated by four
orbits (and seven stellar rotations). Using a bootstrap analysis we find that
flares and star spots reduce the accuracy of transit parameters by up to 10% in
the planet-to-star radius ratio and the accuracy on transit time by 3-4
minutes. Nevertheless, occulted stellar spot features independently confirm the
presence of transit timing variations (TTVs) with an amplitude of at least 4
minutes. We find that the outer companion, AU Mic c may cause the observed
TTVs.
