We report on Keck optical BVRI images and spectroscopy of the companion of the 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 counterpart reveal Balmer lines which confirm that the companion is a Helium–core white dwarf .
We find that the white dwarf has a temperature of T _ { \mathrm { eff } } = 8060 \pm 150 K. Unfortunately , the spectra are of insufficient quality to put a strong constraint on the surface gravity , although the best fit is for low \log g and hence low mass ( \sim 0.2 \mathrm { M } _ { \odot } ) , as expected .
We compare predicted white dwarf cooling ages with the characteristic age of the pulsar and find similar values for white dwarf masses of 0.19 to 0.3 \mathrm { M } _ { \odot } .
These masses would imply a distance of 2.5 to 4 kpc to the system .
The spectroscopic observations also enable us to estimate the mass ratio between the white dwarf and the pulsar .
We find q = 7.5 ~ { } \pm~ { } 2.4 , which is consistent with the current knowledge of white dwarf companions to millisecond pulsars .