  We study the response of the antiferromagnetism of CeAuSb$_2$ to orthorhombic
lattice distortion applied through in-plane uniaxial pressure. The response to
pressure applied along a $\langle 110 \rangle$ lattice direction shows a
first-order transition at zero pressure, which shows that the magnetic order
lifts the $(110)/(1\bar{1}0)$ symmetry of the unstressed lattice. Sufficient
$\langle 100 \rangle$ pressure appears to rotate the principal axes of the
order from $\langle 110 \rangle$ to $\langle 100 \rangle$. At low $\langle 100
\rangle$ pressure, the transition at $T_N$ is weakly first-order, however it
becomes continuous above a threshold $\langle 100 \rangle$ pressure. We discuss
the possibility that this behavior is driven by order parameter fluctuations,
with the restoration of a continuous transition a result of reducing the
point-group symmetry of the lattice.
