So, i have it:
1. The Path to *.dcu must be in the "Library path". Otherwise Delphi does not find the
unit.
2. But: *.pas
must not reside in this directory. Otherwise it will be compiled everytime the main project will be built.
3. The "Search path" mentioned above is not needed for this usage at all. On the contrary: Delphi uses this option to find the sources to be compiled that belong to the project.
Thanks