  We present a photometric $J$-band variability study of GU Psc b, a T3.5
co-moving planetary-mass companion (9-13$M_{\rm{Jup}}$) to a young ($\sim$150
Myr) M3 member of the AB Doradus Moving Group. The large separation between GU
Psc b and its host star (42") provides a rare opportunity to study the
photometric variability of a planetary-mass companion. The study presented here
is based on observations obtained from 2013 to 2014 over three nights with
durations of 5-6 hr each with the WIRCam imager at Canada-France-Hawaii
Telescope. Photometric variability with a peak-to-peak amplitude of $4\pm1$% at
a timescale of $\sim$6 hr was marginally detected on 2014 October 11. No
high-significance variability was detected on 2013 December 22 and 2014 October
10. The amplitude and timescale of the variability seen here, as well as its
evolving nature, is comparable to what was observed for a variety of field T
dwarfs and suggests that mechanisms invoked to explain brown dwarf variability
may be applicable to low-gravity objects such as GU Psc b. Rotation-induced
photometric variability due to the formation and dissipation of atmospheric
features such as clouds is a plausible hypothesis for the tentative variation
detected here. Additional photometric measurements, particularly on longer
timescales, will be required to confirm and characterize the variability of GU
Psc b, determine its periodicity and to potentially measure its rotation
period.
