The initial-to-final mass relationship is the connection between the mass of a white dwarf and the mass of its progenitor in the main sequence. This function is of paramount importance to aspects such as determining ages and distances of globular clusters, constraining the chemical evolution in galaxies, and also understanding the properties of the galactic population of white dwarfs. Despite its relevance, this relation is still poorly constrained. A promising approach to diminish the uncertainties is to study white dwarfs for which external constraints are available. This is the case of white dwarfs in common proper motion pairs. Important information of the white dwarf member can be inferred from the study of the companion, since they were born at the same time and with the same chemical composition. We report new results obtained from spectroscopic observations of both members of several common proper motion pairs composed of a main sequence star (F, G or K type) and a white dwarf. From the fitting of the absorption lines to theoretical models we obtain the effective temperature and the surface gravity of the white dwarf member and, consequently, its mass and cooling time. The determination of the metallicity of the main sequence companion helps us to infer the metallicity of the progenitor of the white dwarf. This procedure allows us to estimate the main sequence lifetime of the white dwarf, and hence, to determine the total age of the system. At that point, we will be able to derive the mass of the main sequence progenitor of the white dwarf and to better establish the initial-to-final mass relationship.