  Dynamical mass estimates down to the planet-mass regime can help to
understand planet formation. We present Atacama Large Millimeter/submillimeter
Array (ALMA) 1.3 mm observations of FW Tau C, a proposed ~10 $M_{\rm Jup}$
planet-mass companion at ~330 au from the host binary FW Tau AB. We spatially
and spectrally resolve the accretion disk of FW Tau C in ${}^{12}$CO (2-1). By
modeling the Keplerian rotation of gas, we derive a dynamical mass of ~0.1
$M_\odot$. Therefore, FW Tau C is unlikely a planet, but rather a low-mass star
with a highly inclined disk. This also suggests that FW Tau is a triple system
consisting of three ~0.1 $M_\odot$ stars.
