Markarian 421 and Markarian 501 are Active Galactic Nuclei and two of
the most prominent sources of very high energetic gamma rays. In contrast to
the predominantly stable Crab Nebula, the two blazars have extremely variable
gamma-ray fluxes. Those could be caused by erratic accretion flow
instabilities, but also by processes leading to quasi periodic oscillations (QPOs).
Recently, hints of such a QPO of Mrk 501 with a period of about 322 days have
been observed with the Large Area Telescope on-board of the Fermi satellite at GeV energies featuring global and local
significances of the signal above 99%.

The First G-APD Cherenkov Telescope (FACT) has monitored a small sample of
known TeV gamma-ray sources including Mrk 421 and Mrk 501 over the last seven
years. Therefore, it is ideally suited for periodicity studies of Active
Galactic Nuclei, as they require unbiased and continuous long-term observations.
Based on measurements of the well-known gamma-ray flux originating from the
Crab Nebula, corrections to the measured light curves are applied. In total,
2500~hours of data for each of the two blazars have been analysed using the
Lomb-Scargle algorithm. The resulting periodogram of Mrk 501 shows a hint of QPO with a similar period as the one observed by Fermi/LAT.