Can a "pure vector" gravitational wave mimic a "pure tensor" one?
  In the general theory of relativity, gravitational waves have two possible
polarizations, which are transverse and traceless with helicity $\pm 2$. Some
alternatives theories contain additional helicity $0$ and helicity $\pm 1$
polarization modes. Here, we consider a hypothetical "pure vector" theory in
which gravitational waves have only two possible polarizations, with helicity
$\pm 1$. We show that if these polarizations are allowed to rotate as the wave
propagates, then for certain source locations on the sky, the strain outputs of
three ideal interferometric gravitational wave detectors can exactly reproduce
the strain outputs predicted by general relativity.
