entry-authors returns a list of entry authors; that is, atom:author children of the entry. A null list designates no authors.
entry-author returns the first entry author, or #f if none.
Each entry must have at least one author. If not present as a child of the entry, it must be present in the entry's source feed element (for aggregated content), or in the feed element itself.
Currently, you must handle this manually. One possibility is:
(or (entry-author E) (feed-author F))