As apparently nobody knows how that is done ( or nobody wnted to answer such a "silly" quesion ), I'd like to enlight you ;-)
In the syslog facility (on my machine it is rsyslog) there are options to define what is logged and where.
However, in /etc/rsyslog.conf I did the following change:
In that way only authentication log enties of warning or higher level are logged. Information about something or someone logging in not.
This solution might be a bit to "global" for some people, as it would not log any successful authentication.
So if anyone in this forum knows a better solution, please enlight me.
In the meanwhile this is at leat a working work around.