Compilation of the values of λ, measured via the RM effect, as a function of the host star effective temperature (see: http://www.astro.keele.ac.uk/jkt/tepcat/rossiter.html). HAT-P-18b is shown as a filled blue dot. For the two objects marked with red squares, the determination of λ is doubtful (see text for details). The dashed red line at T_eff = 6250 K marks the limit above which the mass of the convective shell becomes negligible (Winn et al. 2010a).