One key prediction of General Relativity is that gravitational waves areemitted with a pure spin-2 polarisation. Any extra polarisation mode, spin-1 orspin-0, is consequently considered a smoking gun for deviations from GeneralRelativity. In this paper, we show that the velocity of merging binaries withrespect to the observer gives rise to spin-1 polarisation in the observer frameeven in the context of General Relativity. These are pure projection effects,proportional to the plus and cross polarisations in the source frame, hencethey do not correspond to new degrees of freedom. We demonstrate that thespin-1 modes can always be rewritten as pure spin-2 modes coming from anaberrated direction. Since gravitational waves are not isotropically emittedaround binary systems, this aberration modifies the apparent orientation of thebinary system with respect to the observer: the system appears slightly rotateddue to the source velocity. Fortunately, this bias does not propagate to otherparameters of the system (and therefore does not spoil tests of GeneralRelativity), since the impact of the velocity can be fully reabsorbed into neworientation angles.<br
