  The planets HR8799bc display nearly identical colours and spectra as variable
young exoplanet analogues such as VHS 1256-1257ABb and PSO J318.5-22, and are
likely to be similarly variable. Here we present results from a 5-epoch SPHERE
IRDIS broadband-$H$ search for variability in these two planets. HR 8799b
aperture photometry and HR 8799bc negative simulated planet photometry share
similar trends within uncertainties. Satellite spot lightcurves share the same
trends as the planet lightcurves in the August 2018 epochs, but diverge in the
October 2017 epochs. We consider $\Delta(mag)_{b} - \Delta(mag)_{c}$ to trace
non-shared variations between the two planets, and rule out non-shared
variability in $\Delta(mag)_{b} - \Delta(mag)_{c}$ to the 10-20$\%$ level over
4-5 hours. To quantify our sensitivity to variability, we simulate variable
lightcurves by inserting and retrieving a suite of simulated planets at similar
radii from the star as HR 8799bc, but offset in position angle. For HR 8799b,
for periods $<$10 hours, we are sensitive to variability with amplitude $>5\%$.
For HR 8799c, our sensitivity is limited to variability $>25\%$ for similar
periods.
