The London dispersion force refers to the temporary attractive force that acts between the electrons in two adjacent atoms when the atoms develop temporary dipoles. Dispersion forces act between any two molecules even when other intermolecular forces are in operation as long as the molecules are in close proximity to each other.
Now, CO is polar and the HCN is also polar molecule. Hence, dipole - dipole interaction forces are also in operation and acts between the two molecules in close proximity to each other.
