  1RXS J083842.1$-$282723 is a nearly synchronous magnetic cataclysmic variable
with a simple X-ray light curve. While its orbital period was fairly well
established at $P_{\rm orb}=98.4$ minutes from optical spectroscopy, indirect
estimates of $P_{\rm spin}/P_{\rm orb}$ ranged from 0.90 to 0.96 because the
short X-ray light curves could not determine the beat period to a factor of 2.
We analyze a recent 50 day TESS observation, and ground-based optical
time-series photometry spanning 9 years, that together measure precise beat,
orbit, and spin periods and enable the X-ray and optical modulations to be
phase aligned. Although the X-ray light curves do not distinguish between a
beat period of 16.11 or 32.22 hours, all of the optical evidence favors the
longer value, with complete pole switching of accretion every half beat cycle.
This would require $P_{\rm spin}/P_{\rm orb}=0.952$. Long-term optical
monitoring also shows a decline in accretion rate, and a change in the
beat-folded light curve. It would be useful to obtain a new X-ray/optical
observation of at least 32 hours duration to examine any associated change in
accretion structure, and confirm the spin and beat periods.
