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The planets HR8799bc display nearly identical colours and spectra as variableyoung exoplanet analogues such as VHS 1256-1257ABb and PSO J318.5-22, and arelikely to be similarly variable. Here we present results from a 5-epoch SPHEREIRDIS broadband-$H$ search for variability in these two planets. HR 8799baperture photometry and HR 8799bc negative simulated planet photometry sharesimilar trends within uncertainties. Satellite spot lightcurves share the sametrends as the planet lightcurves in the August 2018 epochs, but diverge in theOctober 2017 epochs. We consider $\Delta(mag)_{b} - \Delta(mag)_{c}$ to tracenon-shared variations between the two planets, and rule out non-sharedvariability in $\Delta(mag)_{b} - \Delta(mag)_{c}$ to the 10-20$\%$ level over4-5 hours. To quantify our sensitivity to variability, we simulate variablelightcurves by inserting and retrieving a suite of simulated planets at similarradii 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 similarperiods.

A High-Contrast Search for Variability in HR 8799bc with VLT-SPHERE