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Is the population of close-in planets orbiting M dwarfs sculpted by thermallydriven escape or is it a direct outcome of the planet formation process? Anumber of recent empirical results strongly suggest the latter. However, theunique architecture of the TOI-1266 system presents a challenge to models ofplanet formation and atmospheric escape given its seemingly "inverted"architecture of a large sub-Neptune ($P_b=10.9$ days, $R_{p,b}=2.62\pm 0.11\,\mathrm{R}_{\oplus}$) orbiting interior to that of the system's smaller planet($P_c=18.8$ days, $R_{p,c}=2.13\pm 0.12\, \mathrm{R}_{\oplus}$). Here wepresent revised planetary radii based on new TESS and diffuser-assistedground-based transit observations, and characterize both planetary masses usinga set of 145 radial velocity measurements from HARPS-N ($M_{p,b}=4.23\pm 0.69\,\mathrm{M}_{\oplus}, M_{p,c}=2.88\pm 0.80\, \mathrm{M}_{\oplus}$). Our analysisalso reveals a third planet candidate ($P_d=32.3$ days, $M_{p,d}\sin{i} =4.59^{+0.96}_{-0.94}\, \mathrm{M}_{\oplus}$), which if real, would form a chainof near 5:3 period ratios, although the system is likely not in a mean motionresonance. Our results indicate that TOI-1266 b and c are among the lowestdensity sub-Neptunes around M dwarfs and likely exhibit distinct bulkcompositions of a gas-enveloped terrestrial ($X_{\mathrm{env},b}=5.5\pm 0.7$%)and a water-rich world (WMF$_c=59\pm 14$%), which is supported by hydrodynamicescape models. If distinct bulk compositions are confirmed through atmosphericcharacterization, the system's unique architecture would represent aninteresting test case of inside-out sub-Neptune formation at pebble traps.

Masses, Revised Radii, and a Third Planet Candidate in the "Inverted" Planetary System Around TOI-1266