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The WASP-98 planetary system represents a rare case of a hot Jupiter hostedby a metal-poor main-sequence star. We present a follow-up study of this systembased on multi-band photometry and high-resolution spectroscopy. Two newtransit events of WASP-98b were simultaneously observed in four passbands(g,r,i,z), using the telescope-defocussing technique, yielding eighthigh-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-resolutionspectrograph, which we used to remeasure its spectral characteristics, inparticular its metallicity. We found this to be very low, Fe/H]=-0.49, butlarger than was previously reported, [Fe/H]=-0.60. We used these newphotometric and spectroscopic data to refine the orbital and physicalproperties of this planetary system, finding that the stellar and planetarymass measurements are significantly larger than those in the discovery paper.In addition, the multi-band light curves were used to construct an opticaltransmission spectrum of WASP-98b and probe the characteristics of itsatmosphere at the terminator. We measured a lower radius at z compared with theother three passbands. The maximum variation is between the r and z bands, hasa confidence level of roughly 6 sigma and equates to 5.5 pressure scaleheights. We compared this spectrum to theoretical models, investigating severalpossible types of atmospheres, including hazy, cloudy, cloud-free, and clearatmospheres with titanium and vanadium oxide opacities. We could not find agood fit to the observations, except in the extreme case of a clear atmospherewith TiO and VO opacities, in which the condensation of Ti and V wassuppressed. As this case is unrealistic, our results suggest the presence of anadditional optical-absorbing species in the atmosphere of WASP-98b, of unknownchemical nature.

An optical transmission spectrum of the transiting hot Jupiter in the metal-poor WASP-98 planetary system