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Here we present an ensemble study of spin-orbit alignment in 51 close doublestar systems. We determine spin-orbit angles, obliquities, in 39 of thesesystems making use of recently improved apsidal motion rate measurements andapsidal motion constants. In the remaining 12 systems researchers haveconstrained spin-orbit alignment by different combinations of measurements ofapsidal motion rates, projected obliquities and stellar inclinations. Of the 51systems 48 are consistent with alignment albeit with some measurements havinglarge uncertainties. A Fisher distribution with mean zero and a concentrationfactor $\kappa = 6.7$ represents this ensemble well. Indeed employing abootstrapping resampling technique we find our data on these 48 systems isconsistent with perfect alignment. We also confirm significant misalignment intwo systems which travel on eccentric orbits and where misalignments have beenreported on before, namely DI Her and AS Cam. The third misaligned system CVVel orbits on a circular orbit. So while there are some glaring exceptions, themajority of close double star systems for which data are available appear to bewell aligned.

astrophysical journal/the astrophysical journal

THE BANANA PROJECT. VI. Close double stars are well aligned with  noticeable exceptions; results from an ensemble study using apsidal motion  and Rossiter-McLaughlin measurements