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We report on Keck optical BVRI images and spectroscopy of the companion ofthe binary millisecond pulsar PSR J0218+4232. A faint bluish (V=24.2, B-V=0.25)counterpart is observed at the pulsar location. Spectra of this counterpartreveal Balmer lines which confirm that the companion is a Helium-core whitedwarf. We find that the white dwarf has a temperature of Teff=8060+-150 K.Unfortunately, the spectra are of insufficient quality to put a strongconstraint on the surface gravity, although the best fit is for low log g andhence low mass (~0.2 Msun), as expected. We compare predicted white dwarfcooling ages with the characteristic age of the pulsar and find similar valuesfor white dwarf masses of 0.19 to 0.3 Msun. These masses would imply a distanceof 2.5 to 4 kpc to the system. The spectroscopic observations also enable us toestimate the mass ratio between the white dwarf and the pulsar. We findq=7.5+-2.4, which is consistent with the current knowledge of white dwarfcompanions to millisecond pulsars.

Temperature and cooling age of the white dwarf companion of PSR J0218+4232