Which fillfactor is used by ALTER INDEX REBUILD? From technet I got the impression, that I either have to explicitly set the wanted fillfactor or when I do not specify it, the default fillfactor 0 is choosen.

For tables I know to get rows added fillfactor 0 isn't a good choice. Is there a way to rebuild an index, so that the last explicitly index is used ? 

BTW: I think that that should be the default setting.
When a new index is built is uses the default fill factor by default, unless a different fillfactor is specified.

When an index is rebuild it will use the same fill factor that was specified when the index was build or last rebuilt unless a different fillfactor is specified.