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

We present new measurements of the RossiterMcLaughlin (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 (Teff= 5700 +/- 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 (Teff= 5750 +/- 100 K), we detect a projected spinorbit angle of ?= 14 degrees.6 +/- 6 degrees.7. WASP-31 (Teff= 6300 +/- 100 K) is found to be well aligned, with a projected spinorbit angle of ?= 2 degrees.8 +/- 3 degrees.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 cool stars (Teff < 6250 K) are preferentially aligned, WASP-31 has little impact on the proposed trend. We bring the total distribution of the true spinorbit alignment angle, ?, up to date, noting that recent results have improved the agreement with the theory of Fabrycky & Tremaine at mid-range angles. We also suggest a new test for judging misalignment using the Bayesian information criterion, according to which WASP-25 bs orbit should be considered to be aligned.