Is it possible to enter the US on VWP, then leave and return on a J1 visa immediately?
On a thread on Tripadvisor, an almost identical question was asked. In response, user Carmen D mentioned that:
I sent an e-mail to CBP INFO Center.
They told me:
"You are able to enter the U.S. with an ESTA. The burden of proof will be on you to convince the CBP officer your intent while your here in the U.S."
And I asked again and:
"You can visit the US with the visa waiver program. You will be OK with a round trip ticket to show you will leave the US before you start as a postdoc with the J1 visa."
So, it looks like you are fine entering the USA on the VWP, but you will be evaluated as a potential overstay risk just like any other person admitted on the VWP. Make sure you are able to show the immigration official why and how you are going to depart the USA according to the terms of the VWP.