  We report on the detection of transit timing variations (TTV) of WASP-161b by
using the combination of TESS data and archival data. The midpoint of the
transits in TESS data are offset by $\sim$67 minutes in Jan. 2019, and
$\sim$203 minutes in Jan. 2021, based on the ephemeris published in previous
work. We are able to reproduce the transit timings from the archival light
curve (SSO-Europa; Jan. 2018) and find SSO-Europa timing is consistent with the
published ephemeris under a constant period assumption. Conversely, we find
that the SSO-Europa transit midpoint indicates a 6.62-minute variation at 4.40
$\sigma$ compared to the prediction obtained from TESS timings, and a constant
orbit period assumption. The TTVs could be modeled with a quadratic function,
yielding a constant period change. The period derivative $\dot{P}$ is
-1.16$\times$10$^{-7}\pm$2.25$\times$10$^{-8}$ days per day (or $-3.65$
s/year), derived from SSO-Europa and TESS timing. Different scenarios,
including a decaying period and apsidal precession can potentially explain
these TTVs but they both introduce certain inconsistencies. We have obtained
CHEOPS observations for two transits in Jan. 2022 to distinguish between
different TTV scenarios. We expect the timing to vary by 5 minutes, compared to
the timing predicted from SSO-Europa and TESS with a constant period
assumption.
