  Here we report large-amplitude transit timing variations (TTVs) for
AU\,Microcopii b and c as detected in combined TESS (2018, 2020) and CHEOPS
(2020, 2021) transit observations. AU Mic is a young planetary system with a
debris disk and two transiting warm Neptunes. A TTV on the order of several
minutes was previously reported for AU Mic b, which was suggested to be an
outcome of mutual perturbations between the planets in the system. In 2021, we
observed AU Mic b (five transits) and c (three transits) with the CHEOPS space
telescope to follow-up the TTV of AU Mic b and possibly detect a TTV for AU Mic
c. When analyzing TESS and CHEOPS 2020--2021 measurements together, we find
that a prominent TTV emerges with a full span of ~23 minutes between the two
TTV extrema. Assuming that the period change results from a periodic process -
such as mutual perturbations - we demonstrate that the times of transits in the
summer of 2022 are expected to be 30--85 minutes later than predicted by the
available linear ephemeris.
