My copy of Exile's Song has only Marion Zimmer Bradley's name on the cover, and the copyright page says:

Copyright © 1996 by Marion Zimmer Bradley All Rights Reserved
Cover art by Romas Kukalis
...
For Arienne Martine-Barnes,
who created the character Margaret Alton,
and worked on this book with me.

In my copy of The Shadow Matrix the copyright says:

Copyright © 1997 by Marion Zimmer Bradley and Adrienne Marline-Barnes.

I don't have a copy of Traitor's Sun so I cannot check it.

I guess after the opriginal publication of Exile's Song it was judged that Martine-Barnes' contribution was large enough to merit co-authorship.

My copy of Exile's Song has only Marion Zimmer Bradley's name on the cover, and the copyright page says:

Copyright © 1996 by Marion Zimmer Bradley All Rights Reserved
Cover art by Romas Kukalis
...
For Arienne Martine-Barnes,
who created the character Margaret Alton,
and worked on this book with me.

In my copy of The Shadow Matrix the copyright says:

Copyright © 1997 by Marion Zimmer Bradley and Adrienne Marline-Barnes.

I don't have a copy of Traitor's Sun so I cannot check it.

I guess after the original publication of Exile's Song it was judged that Martine-Barnes' contribution was large enough to merit co-authorship.