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Context: TOI-2076 is a transiting three-planet system of sub-Neptunesorbiting a bright (G = 8.9 mag), young ($340\pm80$ Myr) K-type star. Although avalidated planetary system, the orbits of the two outer planets wereunconstrained as only two non-consecutive transits were seen in TESSphotometry. This left 11 and 7 possible period aliases for each.  Aims: To reveal the true orbits of these two long-period planets, precisephotometry targeted on the highest-probability period aliases is required.Long-term monitoring of transits in multi-planet systems can also helpconstrain planetary masses through TTV measurements.  Methods: We used the MonoTools package to determine which aliases to follow,and then performed space-based and ground-based photometric follow-up ofTOI-2076 c and d with CHEOPS, SAINT-EX, and LCO telescopes.  Results: CHEOPS observations revealed a clear detection for TOI-2076 c at$P=21.01538^{+0.00084}_{-0.00074}$ d, and allowed us to rule out three of themost likely period aliases for TOI-2076 d. Ground-based photometry furtherenabled us to rule out remaining aliases and confirm the $P=35.12537\pm0.00067$d alias. These observations also improved the radius precision of all threesub-Neptunes to $2.518\pm0.036$, $3.497\pm0.043$, and $3.232\pm0.063$$R_\oplus$. Our observations also revealed a clear anti-correlated TTV signalbetween planets b and c likely caused by their proximity to the 2:1 resonance,while planets c and d appear close to a 5:3 period commensurability, althoughmodel degeneracy meant we were unable to retrieve robust TTV masses. Theirinflated radii, likely due to extended H-He atmospheres, combined with lowinsolation makes all three planets excellent candidates for future comparativetransmission spectroscopy with JWST.

Uncovering the true periods of the young sub-Neptunes orbiting TOI-2076