Investigation of the D s * Ds η (′) and B s * Bs η (′) vertices via QCD sum rules E. Yazıcı, E. Veli Veliev, K. Azizi and H. Sundu Eur. Phys. J. Plus, 128 10 (2013) 113 Published online: 14 October 2013 DOI: 10.1140/epjp/i2013-13113-2