8:34 AM PT -- A source at the chapel where they had the ceremony tells us they'll only perform it with a marriage license, but a source close to Travis and Kourtney tells us they hadn't gotten a license, which wouldn't make it legal.
Sources connected to the two tell us ... prior to Sunday night's Vegas wedding, the two had started working on a prenup, but nothing had been signed. We're told the plan now will be for a postnup -- and should be handled without issue.
Kourtney is reportedly worth around $65 million and Travis around $50 million ... so there's a lot of money on the line.
Prenups and postnups at this level typically separate earnings during the marriage ... that's the way Kim and Kanye handled it. What they bring into the marriage stays as separate property.
TMZ broke the story -- Kourtney and Travis beelined it to a Vegas chapel Sunday night after the Grammys for an Elvis-themed wedding ceremony. We're told they didn't allow the venue to take any photos, bringing their own photog instead as well as their security team.
What's more ... we're told the two came prepared with a marriage license -- which they presented to the chapel's owner -- so while it may have seemed impulsive, there was still some planning behind the scenes.
Our sources say Kourtney and Travis are planning on something more elaborate in the future with close friends and family.