According to the wikipedia articles (Exile's Song, The Shadow Matrix, and Traitor's Sun) the books were co-written with the American author Adrienne Martine-Barnes. Wikipedia claims should always be treated with care, but this information is also confirmed by the isfdb, which notes:
The publications and copyright only credit Marion Zimmer Bradley as the author. However, the Marion Zimmer Bradley Literary Works Trust web site credits Adrienne Martine-Barnes as co-author.
The Marion Zimmer Bradley Literary Works Trust web site can currently be found at this web address (the link provided by the isfdb is now defunct). The page, presumably the most official statement that can be obtained, confirms the authorship:
REDISCOVERY, 1993 (with Mercedes Lackey)
EXILE'S SONG, 1996 (with Adrienne Martine-Barnes)
SHADOW MATRIX, 1998 (by Adrienne Martine-Barnes)
TRAITOR'S SUN, 1999 (by Adrienne Martine-Barnes)
and moreover indicates that while Exile's Song was written with Martine-Barnes, the other two novels appear as being written by Martine-Barnes alone.