We present spectroscopic and photometric observations of the optical counterpart to PSR J1911 - 5958A , a millisecond pulsar located towards the globular cluster NGC 6752 .
We measure radial velocities from the spectra and determine the systemic radial velocity of the binary and the radial-velocity amplitude of the white-dwarf orbit .
Combined with the pulsar orbit obtained from radio timing , we infer a mass ratio of M _ { \mathrm { PSR } } / M _ { \mathrm { WD } } = 7.36 \pm 0.25 .
The spectrum of the counterpart is that of a hydrogen atmosphere , showing Balmer absorption lines up to H12 , and we identify the counterpart as a helium-core white dwarf of spectral type DA5 .
Comparison of the spectra with hydrogen atmosphere models yield a temperature T _ { \mathrm { eff } } = 10090 \pm 150 K and a surface gravity \log g = 6.44 \pm 0.20 cgs .
Using mass-radius relations appropriate for low-mass helium-core white dwarfs , we infer the white-dwarf mass M _ { \mathrm { WD } } = 0.18 \pm 0.02 M _ { \odot } and radius R _ { \mathrm { WD } } = 0.043 \pm 0.009 R _ { \odot } .
Combined with the mass ratio , this constrains the pulsar mass to M _ { \mathrm { PSR } } = 1.40 ^ { +0.16 } _ { -0.10 } M _ { \odot } .
If we instead use the white-dwarf spectrum and the distance of NGC 6752 to determine the white-dwarf radius , we find R _ { \mathrm { WD } } = 0.058 \pm 0.004 R _ { \odot } .
For the observed temperature , the mass-radius relations predict a white-dwarf mass of M _ { \mathrm { WD } } = 0.175 \pm 0.010 M _ { \odot } , constraining the pulsar mass to M _ { \mathrm { PSR } } = 1.34 \pm 0.08 M _ { \odot } .
We find that the white-dwarf radius determined from the spectrum and the systemic radial velocity of the binary are only marginally consistent with the values that are expected if PSR J1911 - 5958A is associated with NGC 6752 .
We discuss possible causes to explain this inconsistency , but conclude that our observations do not conclusively confirm nor disprove the assocation of the pulsar binary with the globular cluster .