  An analysis is presented of the power spectrum of X-ray variability of the
bright Seyfert 1 galaxy Mrk 766 as observed by XMM-Newton. Over the 0.2-10 keV
energy range the power spectral density (PSD) is well-represented by a
power-law with a slope of alpha_low ~ 1 at low frequencies, breaking to a slope
of alpha_hi = 2.8 (-0.4/+0.2) at a frequency f_br ~ 5 x 10^-4 Hz. As has been
noted before this broken power-law PSD shape is similar to that observed in the
Galactic black hole candidate Cygnus X-1. If it is assumed that Mrk 766 shows a
power spectrum similar in form to that of Cyg X-1, and that the break timescale
scales linearly with black hole mass, then the mass of the black hole in Mrk
766 is inferred to be < 5 x 10^5 M_sun. This rather low mass would mean Mrk 766
radiates above the Eddington limit. The coherence between different energy
bands is significantly below unity implying that variations in the different
energy bands are rather poorly correlated. The low coherence can be explained
in the framework of standard Comptonisation models if the properties of the
Comptonising medium are rapidly variable or if there are several distinct
emission sites.
