  WASP-98 is a planetary system containing a hot Jupiter transiting a late-G
dwarf. A fainter star 12 arcsec distant has previously been identified as a
white dwarf, with a distance and proper motion consistent with a physical
association with the planetary system. We present spectroscopy of the white
dwarf, with the aim of determining its mass, radius and temperature and hence
the age of the system. However, the spectra show the featureless continuum and
lack of spectral lines characteristic of the DC class of white dwarfs. We
therefore fitted theoretical white dwarf spectra to the ugriz apparent
magnitudes and Gaia DR2 parallax of this object in order to determine its
physical properties and the age of the system. We find that the system is old,
with a lower limit of 3.6 Gyr, but theoretical uncertainties preclude a precise
determination of its age. Its kinematics are consistent with membership of the
thick disc, but do not allow us to rule out the thin-disc alternative. The old
age and low metallicity of the system suggest it is subject to an
age-metallicity relation, but analysis of the most metal-rich and metal-poor
transiting planetary systems yields only insubstantial evidence of this. We
conclude that the study of bound white dwarfs can yield independent ages to
planetary systems, but such analysis may be better-suited to DA and DB rather
than DC white dwarfs.
