Rossiter-McLaughlin effect measurements for WASP-16, WASP-25 and WASP-31

We present new measurements of the Rossiter-McLaughlin (RM) effect for three Wide Angle Search for transiting Planets (WASP) planetary systems, WASP-16, WASP-25 and WASP-31, from a combined analysis of their complete sets of photometric and spectroscopic data. We find a low-amplitude RM effect for WASP-16 (T$_eff$= 5700 plusmn 150 K), suggesting that the star is a slow rotator and thus of an advanced age, and obtain a projected alignment angle of ?. For WASP-25 (T$_eff$= 5750 plusmn 100 K), we detect a projected spin-orbit angle of $= 14deg.6 plusmn 6deg.7. WASP-31 (T$_eff$= 6300 plusmn 100 K) is found to be well aligned, with a projected spin-orbit angle of $= 2deg.8 plusmn 3deg.1. A circular orbit is consistent with the data for all three systems, in agreement with their respective discovery papers. We consider the results for these systems in the context of the ensemble of RM measurements made to date. We find that whilst WASP-16 fits the hypothesis of Winn et al. that lsquocoolrsquo stars (T$_eff$ $ 6250 K) are preferentially aligned, WASP-31 has little impact on the proposed trend. We bring the total distribution of the true spin-orbit alignment angle, $, up to date, noting that recent results have improved the agreement with the theory of Fabrycky amp Tremaine at mid-range angles. We also suggest a new test for judging misalignment using the Bayesian information criterion, according to which WASP-25 brsquos orbit should be considered to be aligned. Based on observations made using the CORALIE high-resolution echelle spectrograph mounted on the 1.2-m Euler Swiss Telescope and the HARPS high-resolution echelle spectrograph mounted on the ESO 3.6-m (under proposals 084.C-0185 and 085.C-0393), both at the ESO La Silla observatory.