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 for Exile's Song:
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.
while The Shadow Matrix and Traitor's Sun have similar but slightly different entries, noting that for these novels Martine-Barnes was credited in in the copyright (though not on the cover or title pages), probably indicating a greater role in their creation. The isfdb entry for Traitor's Sun additionally includes the comment: "The book was published in January-1999 and MZB's death was not until September-1999 meaning they may have worked together in some form."
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.
