  The detection of quasi-periodic oscillations (QPOs) in the light curves of
active galactic nuclei (AGNs) can provide insights into the physics of the
super-massive black-holes (SMBHs) powering these systems and could represent a
signature of the existence of SMBH binaries, setting fundamental constraints on
SMBH evolution in the Universe. The identification of long term QPOs,
characterized by periods on the order of several months to years, is
particularly challenging and can only be achieved via all-sky monitoring
instruments that provide unbiased, continuous light-curves of astrophysical
objects. The Fermi-LAT satellite, thanks to its monitoring observing strategy,
is an ideal instrument for such a goal. Here, we aim to identify QPOs in the
$\gamma$-ray light-curves of the brightest AGNs within the Fermi-LAT catalog.
We analyzed the light curves of the 35 brightest Fermi-LAT AGNs, including data
from the beginning of the Fermi mission (August 2008) to April 2021, with
energies from 100 MeV to 300 GeV. Two time binnings were investigated, 7 and 30
days. The search for quasi-periodic features was then performed using the
continuous wavelet transform. The significance of the result was tested via
Monte Carlo simulations of artificial light curves with the same power spectral
density and probability distribution function as the original light curves. The
significances were then corrected for the look-elsewhere effect and provided as
post-trials. We identified 24 quasars with candidate QPOs. Several of our
candidates coincide with previous claims in the literature, namely: PKS
0537-441, S5 0716+714, Mrk 421, B2 1520+31, and PKS 2247-131. All our
candidates are transient. The most significant multi-year QPO, with a period of
about 1100 days, was observed in the quasar S5 1044+71. It is reported here for
the first time.
