The Collaborative International Dictionary of English v.0.48:

coauthor \coauthor\ n.
   a writer who collaborates with others in writing something.

   Syn: joint author.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

co-author \co-author\ v. t.
   1. to to be an author of, together with another author or
      authors; to author jointly with others.
      [WordNet 1.5]

   2. to exist together at the same time.

   Syn: coexist.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

coauthor
    n 1: a writer who collaborates with others in writing something
         [syn: coauthor, joint author]