With a very simple interpolation procedure, a number of fundamental spectroscopic constants (ground-state energies, equilibrium distances, force constants, and frequencies) for asymmetric alkali molecules and alkali molecular ions are estimated, starting from the corresponding values of the symmetric molecules and ions. For the quantities for which there are data available in the literature, good agreement is found both with experimental data and theoretical calculations. In general, the estimated spectroscopic constants may be considered sufficiently reliable to be safely used in applications such as the construction of potential-energy curves.