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The measurement of the Rossiter-McLaughlin effect for transiting exoplanetsplaces constraints on the orientation of the orbital axis with respect to thestellar spin axis, which can shed light on the mechanisms shaping the orbitalconfiguration of planetary systems. Here we present the interesting case of theSaturn-mass planet HAT-P-18b, which orbits one of the coolest stars for whichthe Rossiter-McLaughlin effect has been measured so far. We acquired aspectroscopic time-series, spanning a full transit, with the HARPS-Nspectrograph mounted at the TNG telescope. The very precise radial velocitymeasurements delivered by the HARPS-N pipeline were used to measure theRossiter-McLaughlin effect. Complementary new photometric observations ofanother full transit were also analysed to obtain an independent determinationof the star and planet parameters. We find that HAT-P-18b lies on acounter-rotating orbit, the sky-projected angle between the stellar spin axisand the planet orbital axis being lambda=132 +/- 15 deg. By joint modelling ofthe radial velocity and photometric data we obtain new determinations of thestar (M_star = 0.770 +/- 0.027 M_Sun; R_star= 0.717 +/- 0.026 R_Sun;Vsin(I_star) = 1.58 +/- 0.18 km/s) and planet (M_pl = 0.196 +/- 0.008 M_J; R_pl= 0.947 +/- 0.044 R_J) parameters. Our spectra provide for the host star aneffective temperature T_eff = 4870 +/- 50 K, a surface gravity of log(g_star) =4.57 +/- 0.07 cm/s, and an iron abundance of [Fe/H] = 0.10 +/- 0.06. HAT-P-18bis one of the few planets known to transit a star with T_eff < 6250 K on aretrograde orbit. Objects such as HAT-P-18b (low planet mass and/or relativelylong orbital period) most likely have a weak tidal coupling with their parentstars, therefore their orbits preserve any original misalignment. As such, theyare ideal targets to study the causes of orbital evolution in coolmain-sequence stars.

The GAPS Programme with HARPS-N at TNG. III: The retrograde orbit of HAT-P-18b