  The WASP-98 planetary system represents a rare case of a hot Jupiter hosted
by a metal-poor main-sequence star. We present a follow-up study of this system
based on multi-band photometry and high-resolution spectroscopy. Two new
transit events of WASP-98b were simultaneously observed in four passbands
(g,r,i,z), using the telescope-defocussing technique, yielding eight
high-precision light curves with point-to-point scatters of less than 1 mmag.
We also collected three spectra of the parent star with a high-resolution
spectrograph, which we used to remeasure its spectral characteristics, in
particular its metallicity. We found this to be very low, Fe/H]=-0.49, but
larger than was previously reported, [Fe/H]=-0.60. We used these new
photometric and spectroscopic data to refine the orbital and physical
properties of this planetary system, finding that the stellar and planetary
mass measurements are significantly larger than those in the discovery paper.
In addition, the multi-band light curves were used to construct an optical
transmission spectrum of WASP-98b and probe the characteristics of its
atmosphere at the terminator. We measured a lower radius at z compared with the
other three passbands. The maximum variation is between the r and z bands, has
a confidence level of roughly 6 sigma and equates to 5.5 pressure scale
heights. We compared this spectrum to theoretical models, investigating several
possible types of atmospheres, including hazy, cloudy, cloud-free, and clear
atmospheres with titanium and vanadium oxide opacities. We could not find a
good fit to the observations, except in the extreme case of a clear atmosphere
with TiO and VO opacities, in which the condensation of Ti and V was
suppressed. As this case is unrealistic, our results suggest the presence of an
additional optical-absorbing species in the atmosphere of WASP-98b, of unknown
chemical nature.
