  We have constructed an optical photometer with microsecond time resolution,
which is currently being operated on one of the H.E.S.S. telescopes. H.E.S.S.
is an array of four Cherenkov telescopes, each with a 107 m^2 mirror, located
in the Khomas highland in Namibia. In its normal mode of operation H.E.S.S.
observes Cherenkov light from air showers generated by very high energy
gamma-rays in the upper atmosphere. Our detector consists of seven
photomultipliers, one in the center to record the lightcurve from the target
and six concentric photomultipliers as a veto system to reject disturbing
signals e.g. from meteorites or lightning at the horizon. The data acquisition
system has been designed to continuously record the signals with zero deadtime.
The Crab pulsar has been observed to verify the performance of the instrument
and the GPS timing system. Compact galactic targets were observed to search for
flares on timescales of a few microseconds to ~ 100 milliseconds. The design
and sensitivity of the instrument as well as the data analysis method are
presented.
