Marc Anthony & Jennifer Lopez -- Reunited ... But

Marc Anthony and Jennifer Lopez were together in Puerto Rico last week -- but we're told there's no reconciliation here ... it was was strictly business.

The ex-couple went to the Caribe Hilton in San Juan, according to the Puerto Rican website Primera Hora ... where they were filming their upcoming Latin talent competition show, "¡Q'Viva! The Chosen."

We're told they watched auditions last Tuesday night and seemed like they were having fun together.