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We present an atmospheric analysis of LTT 9779 b, a rare planet situated inthe hot Neptune desert, that has been observed with HST WFC3 G102 and G141. Thecombined transmission spectrum, which covers 0.8 - 1.6 $\mu$m, shows a gradualincrease in transit depth with wavelength. Our preferred atmospheric modelshows evidence for H$_{\rm 2}$O, CO$_{\rm 2}$ and FeH with a significance of3.1 $\sigma$, 2.4 $\sigma$ and 2.1 $\sigma$, respectively. In an attempt toconstrain the rate of atmospheric escape for this planet, we search for the1.083 $\mu$m Helium line in the G102 data but find no evidence of excessabsorption that would indicate an escaping atmosphere using this tracer. Werefine the orbital ephemerides of LTT 9779 b using our HST data andobservations from TESS, searching for evidence of orbital decay or apsidalprecession, which is not found. The phase-curve observation of LTT 9779 b withJWST NIRISS should provide deeper insights into the atmosphere of this planetand the expected atmospheric escape might be detected with further observationsconcentrated on other tracers such as Lyman $\alpha$.

Characterising a World Within the Hot Neptune Desert: Transit Observations of LTT 9779 b with HST WFC3