There's been some rumours in recent days from some of the usual suspects that Williams have agreed to Porsche buying a stake in the team and they will become the Porsche factory team (I'd assume they'd keep the Williams name somewhere too)
I assume if they do they'll now just share the Audi engine given that they were expecting to just re-badge a Red Bull engine.
Porsche and Audi are part of the same group, so that would make sense.