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-98 b were simultaneously observed in four passbands ( g ^ { \prime } ,r ^ { \prime } ,i ^ { \prime } ,z ^ { \prime } ) , 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 \pm 0.10 , but larger than was previously reported , { [ Fe / H ] } = -0.60 \pm 0.19 .
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-98 b and probe the characteristics of its atmosphere at the terminator .
We measured a lower radius at z ^ { \prime } compared with the other three passbands .
The maximum variation is between the r ^ { \prime } and z ^ { \prime } 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-98 b , of unknown chemical nature .