We analyze the proving activity of three students of a univer- sity geometry course, when they were working collaboratively to solve a problem. Underlying the solution process is the production of a theorem within a determined theory. With the purpose of describing the students' participation, the analysis concentrates in identifying and keeping track of the mathematical ideas that emerge and in identifying, in the students' ac- tions, the three aspects that, according to Habermas, characterize a rational behavior (teleological, epistemic and communicative). The findings permit us to affirm that undergraduate students can produce a theorem.