In the movie Watchmen (2009), we can see posters of Batman during the credit sequence in beginning. Also, 'Superman' was also mentioned during the flashback scene of Dr. Manhattan. So, the DC characters do exist in Watchmen universe as fictional comicbook characters
(as established in the ongoing doomsday clock series).
Are there any DC characters ever mentioned in the comic-book version (before the doomsday clock series) ?