  Millimeter wavelength observations of Class II protoplanetary disks often
display strong emission from hydrocarbons and high CS/SO values, providing
evidence that the gas-phase C/O ratio commonly exceeds 1 in their outer
regions. We present new NOEMA observations of CS $5-4$, SO $7_6-6_5$ and
$5_6-4_5$, C$_2$H $N=3-2$, HCN $3-2$, HCO$^+$ $3-2$, and H$^{13}$CO$^+$ $3-2$
in the DR Tau protoplanetary disk at a resolution of $\sim0.4''$ (80 au).
Estimates for the disk-averaged CS/SO ratio range from $\sim0.4-0.5$, the
lowest value reported thus far for a T Tauri disk. At a projected separation of
$\sim180$ au northeast of the star, the SO moment maps exhibit a clump that has
no counterpart in the other lines, and the CS/SO value decreases to $<0.2$ at
its location. Thermochemical models calculated with DALI indicate that DR Tau's
low CS/SO ratio and faint C$_2$H emission can be explained by a gas-phase C/O
ratio that is $<1$ at the disk radii traced by NOEMA. Comparisons of DR Tau's
SO emission to maps of extended structures traced by $^{13}$CO suggest that
late infall may contribute to driving down the gas-phase C/O ratio of its disk.
