  We present photometric reverberation mapping of the narrow-line Seyfert 1
galaxy ESO399-IG20 performed with the robotic 15 cm telescope VYSOS-6 at the
Cerro Armazones Observatory. Through the combination of broad- and narrow-band
filters we determine the size of the broad-line emitting region (BLR) by
measuring the time delay between the variability of the continuum and the
H_alpha emission line. We use the flux variation gradient method to separate
the host galaxy contribution from that of the active galactic nucleus (AGN),
and to calculate the 5100A luminosity L_AGN of the AGN. Both measurements
permit us to derive the position of ESO399-IG20 in the BLR size -- AGN
luminosity R_BLR-L_AGN^0.5 diagram. We infer the basic geometry of the BLR
through modelling of the light curves. The pronounced sharp variability
patterns in both the continuum and the emission line light curves allow us to
reject a spherical BLR geometry. The light curves are best fitted by a
disk-like BLR seen nearly face-on with an inclination angle of 6+/-3 degrees
and with an extension from 16 to 20 light days.
