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

Context: TOI-2076 is a transiting three-planet system of sub-Neptunes
orbiting a bright (G = 8.9 mag), young ($340\pm80$ Myr) K-type star. Although a
validated planetary system, the orbits of the two outer planets were
unconstrained as only two non-consecutive transits were seen in TESS
photometry. This left 11 and 7 possible period aliases for each.
Aims: To reveal the true orbits of these two long-period planets, precise
photometry targeted on the highest-probability period aliases is required.
Long-term monitoring of transits in multi-planet systems can also help
constrain 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 of
TOI-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 the
most likely period aliases for TOI-2076 d. Ground-based photometry further
enabled 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 three
sub-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 signal
between 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, although
model degeneracy meant we were unable to retrieve robust TTV masses. Their
inflated radii, likely due to extended H-He atmospheres, combined with low
insolation makes all three planets excellent candidates for future comparative
transmission spectroscopy with JWST.