Evidence for orbital motion of CW Leonis from ground-based astrometry

Recent ALMA observations indicate that CW Leo, the closest carbon-rich AGB star to the Sun, might have a low-mass stellar companion. We present archival ground-based astrometric measurements of CW Leo obtained within the context of the Torino Parallax Program and with $>6$ yr ($1995 - 2001$) of time baseline. The residuals to a single-star solution show significant curvature, and they are strongly correlated with the well-known $I$-band photometric variations due to stellar pulsations. We describe successfully the astrometry of CW Leo with a variability-induced motion (VIM) + acceleration model. We obtain proper motion and parallax of the center-of-mass of the binary, the former in fair agreement with recent estimates, the latter at the near end of the range of inferred distances based on indirect methods. The VIM + acceleration model results allow us to derive a companion mass in agreement with that inferred by ALMA, they point towards a somewhat longer period than implied by ALMA, but are not compatible with much longer period estimates. These data will constitute a fundamental contribution towards the full understanding of the orbital architecture of the system when combined with Gaia astrometry, providing a $\sim 25$\,yr time baseline.