Top : phase-folded white light curve of 2M1721+33 using the 2.6±0.1 hour period reported by Metchev et al. (2015). The photometric points from each HST orbit are colour-coded and the best-fit celerité2 model is plotted in black. Using this period for phasefolding does not give a clean light curve. Bottom : phase-folded white light curve according to the period of ${4.9}_{-0.2}^{+0.4}$ hours from this work. While this observation does not cover more than 1.5 rotations, the phase-folded regions (orbits 1 and 4, and orbits 2 and 5) overlap with each other in both the HST data and the celerité2 model. This cleaner phase-folded light curve, along with Figure 2 motivated us to adopt this new period for 2M1721+33.