(Top): possible histories of the spin period of the HATS-18 system, based on our model for tidal evolution. Each curve shows the calculated evolution for a set of system parameters drawn randomly from distributions representing the observational uncertainties. The squares are random samples of the current spin period and system age. The dashed line is the Skumanich relation (﹩{P}_{\mathrm{spin}}\propto {t}^{1/2}﹩), which would apply in the absence of a planet. (Bottom): same, for the orbital period. To determine constraints on ﹩{Q}_{\star }^{{\prime} }﹩ we marginalize over the uncertainties in all the observed parameters.