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We investigate the radial velocity (RV) variability and spectroscopicbinarity of 19 Galactic long-period ($P_{\rm{puls}} \gtrsim 10$ d) classicalCepheid variable stars whose trigonometric parallaxes are being measured usingthe Hubble Space Telescope and Gaia. Our primary objective is to constrainpossible parallax error due to undetected orbital motion. Using $>1600$high-precision RVs measured between 2011 and 2016, we find no indication oforbital motion on $\lesssim 5$ yr timescales for 18 Cepheids and determineupper limits on allowed configurations for a range of input orbital periods.The results constrain the unsigned parallax error due to orbital motion to $< 2\%$ for 16 stars, and $< 4 \%$ for 18. We improve the orbital solution of theknown binary YZ Carinae and show that the astrometric model must take intoaccount orbital motion to avoid significant error ($\sim \pm 100 \mu$arcsec).We further investigate long-timescale ($P_{\rm{orb}} > 10$ yr) variations inpulsation-averaged velocity $v_\gamma$ via a template fitting approach usingboth new and literature RVs. We discover the spectroscopic binarity of XZ Carand CD Cyg, find first tentative evidence for AQ Car, and reveal KN Cen'sorbital signature. Further (mostly tentative) evidence of time-variable$v_\gamma$ is found for SS CMa, VY Car, SZ Cyg, and X Pup. We briefly discussconsiderations regarding a vetting process of Galactic Leavitt law calibratorsand show that light contributions by companions are insignificant for mostdistance scale applications.

the astrophysical journal. supplement series/astrophysical journal. supplement series

Vetting Galactic Leavitt Law Calibrators using Radial Velocities: On the  Variability, Binarity, and Possible Parallax Error of 19 Long-period Cepheids