The dissimilar chemical composition of the planet-hosting stars of the XO-2 binary system - Ramírez, I; Khanal, S; Aleo, P; Sobotka, A; Liu, F; Casagrande, L; Moreno, Jorge Luiz Meléndez; Yong, D; Lambert, D. L; Asplund, M (The Astrophysical Journal, 2015)