Comparing DNR and WWKL (with Klaus Ambos-Spies, Steffen Lempp, and Theodore A. Slaman).
Journal of Symbolic Logic 69 (2004), no. 4, 1089—1104.

This paper (which is mentioned in Wikipedia under Reverse Mathematics) separated DNR from WWKL. It was mostly written while all four authors were in Heidelberg during 2002-2003.