> If you wanted to exclude /dir/, but include /dir/subdir/ 
you could try the following, but there must be links 
elsewhere that point to those lower levels of /dir/ else 
you will not be able to mirror them
> -*/dir/*
> +*/dir/*/*

Actually I wanted the opposite:
the files in the base directory 
the files in some subdirectories
none of the files in the other subdirs

I have done it now with:
+*/dir/*
-*/dir/*/*
+*/dir/subdir_a/*
+*/dir/subdir_b/*

This seems to be what I want :-)

Thank for both answers

Best regards
Juergen