In order to interpret precise measurements of molecular properties the finite
nuclear mass corrections to the Born-Oppenheimer approximation have to be
accounted for. It is shown that they can be obtained systematically in the
perturbative approach. The formulae for the leading corrections to the
relativistic contribution to energy, the transition electric dipole moment, the
electric polarizability, and the magnetic shielding constant are presented.Comment: 24 pages, submitted to Phys. Rev.