  We present the Apache Point Observatory BG40 broadband and simultaneous
Gemini $r$-band and $i$-band high-speed follow-up photometry observations and
analysis of the 40.5 minute period eclipsing detached double-degenerate binary
SDSS J082239.54$+$304857.19. Our APO data spans over 318 days and includes 13
primary eclipses, from which we precisely measure the system's orbital period
and improve the time of mid-eclipse measurement. We fit the light curves for
each filter individually and show that this system contains a low-mass DA white
dwarf with radius $R_A=0.031\pm0.006~{\rm R_\odot}$ and a
$R_B=0.013\pm0.005~{\rm R_\odot}$ companion at an inclination of
$i=87.7\pm0.2^\circ$. We use the best-fitting eclipsing light curve model to
estimate the temperature of the secondary star as $T_{\rm eff}=5200\pm100~{\rm
K}$. Finally, while we do not record significant offsets to the expected time
of mid-eclipse caused by the emission of gravitational waves with our 1-year
baseline, we show that a $3\sigma$ significant measurement of the orbital decay
due to gravitational waves will be possible in 2023, at which point the eclipse
will occur about $8$ seconds earlier than expected.
