LETTER TO THE EDITOR: Grammian N-soliton solutions of a coupled KdV system

From the Lax pair and the binary Darboux transformation of a coupled Korteweg-de Vries system, we show that its nonlinear superposition formula is identical to that obtained for the Kaup-Kupershmidt equation. Therefore, the N-soliton solution can be associated with a determinant of the Gram type.