Welcome to the boards, ZapBrannigan!
In the second year, Barbara Bain had it written into Catherine Schell's contract that she never appear as "Catherine Schell" - only as Maya or a totally different actress if she were to play a human female. Nobody could be prettier than Barbara Bain (no comment).
I can understand protecting the size of your role (although it greatly reduces the series' storytelling latitude), but protecting your status as the best-looking is a little bit sad. And forcing clauses into someone else's contract really smells.
That was sad--particularly since 1970s Bain had aged considerably since her Mission: Impossible
days, and was not eye candy for Space: 1999
viewers. Schell was the breakout star of the show, and instead of focusing on something which could not be controlled (Schell's beauty), Bain should have upped her legitimate acting talents to stand out in the drama department.