they can be detected out to large distances in relatively shallow surveys. These low mass, old (their ages are typically in excess of 10 Gyr) stars are ideal tracers of the Galactic halo, and, indeed, RRL have been used extensively in the literature to study the stellar halo (e.g. @vivas06 [@watkins09; @sesar10; @simion14; @fiorentino15]).

In this work, we use a sample of type AB RRL stars from the Catalina Sky Survey [@drake13a; @drake13b; @torrealba15] to infer the rotation signal of the Milky Way stellar halo. This survey has amassed a large number ($N \sim 22,700$) of RRL stars over 33,000 deg$^2$ of the sky, with distances in excess of 50 kpc. The RRL sample is matched to the SDSS-*Gaia* proper motion catalog by searching the nearest neighbours within $10\arcsec$. Our resulting sample contains $N=8590$ RRL stars with measured 3D positions, photometric metallicities (derived using Eqn. 7 from @torrealba15) and proper motions. The distribution of this sample on the sky in Equatorial coordinates is shown in Fig. \[fig:sdss\_rrl\]. When evaluating the Galactic velocity components of the RRL stars, the random proper motion errors (derived in Section \[sec:pms\]) dominate over the distance errors (typically $\sim 7\%$ see e.g. @simion14), so we can 