stable and robust catalog of proper motions. Halo star tracers that have previously been identified in the literature are cross-matched with this new proper motion catalog to create a sample of halo stars with 2/3D kinematics.

The paper is arranged as follows. In Section \[sec:pms\] we introduce the SDSS-*Gaia* proper motion catalogue and investigate the statistical and systematic uncertainties in these measurements using spectroscopically confirmed QSOs. Our halo star samples are described in Section \[sec:samples\], and we provide further validation of our proper motion measurements by comparison with models and observations of the Sagittarius stream in Section \[sec:sgr\]. In Section \[sec:like\], we introduce our rotating stellar halo model and apply a likelihood analysis to RRL, BHB and K giant halo star samples. We compare our results with state-of-the-art simulations in Section \[sec:sims\], and re-evaluate our expectations for the stellar halo spin. Finally, we summarise our main conclusions in Section \[sec:conc\].

SDSS-*Gaia* Proper Motions {#sec:pms}
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The aim of this work is to infer the average rotation signal of the Galactic halo using a newly calibrated SDSS-*Gaia* catalog. This catalog (described below) is robust to systematic biases, which is vital in order to measure a rotation signal. Indeed, even with large 