Where to mention your publication

[JKL]

Where in an application composed of a writing sample, LOR, SOP, GRE, and transcript would you mention an undergrad publication?

[fuzzylogician]

It should come up in the SOP, assuming it's at least somewhat related to your current/future interests. I'd imagine at least one LOR writer would be aware of it and write about it in his/her letter. And you could use it as a writing sample, but of course you don't have to (especially if it's co-authored and it's not clear what your contribution is as opposed to other team members*).

* This advice is subject to variation by field and since you didn't mention yours, take this with a grain of salt.