of a black hole or neutron star in an X-ray binary (XRB) system [@vdk00]. Consequently, QPOs have been used in galactic XRBs to introduce important constraints on the masses of the central black holes of these systems.

Previous work has revealed that AGN and XRBs are alike: noise power spectra have shown that similar physical processes may be underlying the X-ray variability in both [@ede99; @utt02; @mar03; @vau03; @mchar04; @mchar05]. Taking this resemblance into account and assuming that accretion onto a stellar-mass black hole is comparable to accretion onto a supermassive black hole, one would expect some AGN to exhibit QPOs similar to those observed in XRBs. In supermassive black holes ($10^6$-$10^9$ M$_\sun$), these QPOs would be at much lower frequencies than those we find in stellar-mass black holes ($\sim$ 10 M$_\sun$). Low frequency quasi-periods (LF QPOs) in XRBs range from 50 mHz to 30 Hz; scaling from a $\sim 1$ Hz QPO in a 10 $M_\sun$ XRB, a LF QPO in an AGN would occur on timescales of days to months [@vau05], too long to be detectable for the AGN in our sample. On the other hand, high frequency QPOs (HF QPOs) in XRBs have values of $\geq$ 100 