Ben Affleck and Jennifer Lopez Hold Hands Arriving at Music Studio

Ben and Jen aren't married -- they're not even engaged, yet -- but they are sure acting like a couple that already tied the knot.

B&J walked hand-in-hand to a music studio Saturday ... presumably JLo's recording new tunes. This follows spending Thanksgiving together, along with JLo's twins, Max and Emme.

Ben did double duty, also chowing down with his kids, Violet, Seraphina and Samuel.

Sources with direct knowledge tell TMZ ... Ben and Jen have been looking for a house together in the L.A. area, but so far they haven't sealed the deal. Nonetheless, they are not only spending time together, but they are also traveling all over the world together.

What's going to be interesting ... whether Ben and Jen get all the kids together over the holiday weekend. Since they're looking for a house together, that's probably already happened.