  Stellar masses are fundamental but often difficult to measure. Thanks to the
Atacama Large Millimeter Array (ALMA) and $Gaia$, dynamical masses of pre-main
sequence stars can be precisely measured using the Keplerian rotation of
protoplanetary disks. We used ALMA CO(2-1) and CO(3-2) observations of CT Cha
and DS Tau to determine their masses by modeling the geometry, kinematics, and
physical properties of their disks with a Bayesian-based radiative transfer
modeling code $pdspy$. We found that the posterior distributions of the masses
from the two transitions are inconsistent at the 2-4 $\sigma$ level. These
systematic errors may originate from assumptions in the disk model, or perhaps
the modest spatial or spectral resolutions used in this study. Regardless, this
indicates that dynamical mass measurements using disk kinematics should be
treated with caution when using only a single transition line because of these
systematic errors.
