  The BL Lac Object Mkn 501 was in a state of high activity in the TeV range in
1997. During this time Mkn 501 was observed by all Cherenkov-Telescopes of the
HEGRA-Collaboration. Part of the data were also taken during moonshine thus
providing a nearly continuous coverage for this object in the TeV-range. We
have carried out a QPO analysis and found evidence for a 23 day periodicity. We
applied the same analysis on the 'data by dwell' x-ray lightcurve from the
RXTE/ASM database and found also evidence for the 23 day periodicity. The
combined probability was P = 2.8 *10^-4.
