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


  • 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.



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 (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 inline image. For WASP-25 (Teff= 5750 ± 100 K), we detect a projected spin–orbit angle of λ= 14°.6 ± 6°.7. WASP-31 (Teff= 6300 ± 100 K) is found to be well aligned, with a projected spin–orbit angle of λ= 2°.8 ± 3°.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 spin–orbit alignment angle, ψ, 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 b’s orbit should be considered to be aligned.