Coming back to the US on an ESTA after having stayed 89 days earlier this year. What documents can I prepare to make border control easier?

Upvote:5

It is always difficult to guess what CBP officers will do, however:

The objective is for them to prevent people from actually living in the US, and more importantly from working in the US, without the appropriate visa
Contrary to the Schengen Area which has a very explicit rule (the 90/180 rule), the rule in the US (and similarly in the UK and a number of other countries) is a lot more informal, and subject to the appreciation of the officer.
Back-to-back visits up to the limit (90 days on a ESTA) are clearly a red flag, but shorter visits, or spaced visits are much less of an issue. The usual rule of thumb is to stay out as long as you've stayed, but it's by far not a hard and fast rule.
Since you actually have an activity in France (ongoing PhD), a valid reason to come to the US (a conference), if you do indeed plan to stay only for the duration of the conference (plus a couple days before and/or after), it's very unlikely you'll have any problems.
Don't try to extend the stay too much beyond that, though in my very humble opinion up to 2 or 3 weeks should be OK, but the longer you're staying, the more questions this can raise.
Make sure you have a confirmed return ticket.
Make sure you have your registration for the conference, itinerary, hotel bookings.
As you are presenting at the conference, anything that shows you're not getting paid for it (you're actually probably paying to do so!) could be useful.

jcaron 63479

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