  We present the results of a two year optical continuum photometric
reverberation mapping campaign carried out on the nucleus of the Seyfert-1
galaxy Mrk509. Specially designed narrow-band filters were used in order to
mitigate the line and pseudo-continuum contamination of the signal from the
broad line region, while allowing for high-accuracy flux-calibration over a
large field of view. We obtained light curves with a sub-day time sampling and
typical flux uncertainties of $1\%$. The high photometric precision allowed us
to measure inter-band continuum time delays of up to $\sim 2$ days across the
optical range. The time delays are consistent with the relation $\tau \propto
\lambda^{4/3}$ predicted for an optically thick and geometrically thin
accretion disk model. The size of the disk is, however, a factor of 1.8 larger
than predictions based on the standard thin-disk theory. We argue that, for the
particular case of Mrk509, a larger black hole mass due to the unknown geometry
scaling factor can reconcile the difference between the observations and
theory.
