You can look at what the YC app entails if you click on apply at the bottom of most HN pages. If they filed it together there is definitely talk about equity and ownership of the company among founders. founder required to have at least 10% ownership. IANAL so I don't know how binding applying to an incubator is if you don't have a partnership agreement filled out prior to the application. Without it in writing explicitly it's very gray. I imagine it's not as cut and dry as it seems; especially with legal definitions and practices sometimes going against common sense.
I'm not sure about whether the YC documents become binding on the ownership of the company, but it would indicate that YC has had knowledge of another owner for awhile.
Yeah, I doubt it becomes binding but it could show evidence of intent in the case of undocumented partnerships like it seems they were before incorporating. A valid partnership can be formed without paperwork(though things like this can happen without proper contracts in place).
