Masses, revised radii, and a third planet candidate in the 'Inverted' planetary system around TOI-1266

Is the population of close-in planets orbiting M dwarfs sculpted by thermally driven escape or is it a direct outcome of the planet formation process? A number of recent empirical results strongly suggest the latter. However, the unique architecture of the TOI-1266 system presents a challenge to models of planet formation and atmospheric escape given its seemingly 'inverted' architecture of a large sub-Neptune (P-b = 10.9 d, R-p,R-b = 2.62 +/- 0.11 R-circle plus) orbiting interior to that of the system's smaller planet (P-c = 18.8 d, R-p,R-c = 2.13 +/- 0.12 R-circle plus). Here, we present revised planetary radii based on new TESS and diffuser-assisted ground-based transit observations, and characterize both planetary masses using a set of 145 radial velocity measurements from HARPS-N (M-p,M-b = 4.23 +/- 0.69 M-circle plus, M-p,M-c = 2.88 +/- 0.80 M-circle plus). Our analysis also reveals a third planet candidate (P-d = 32.3 d, M-p,M-d sin i = 4.59(-0.94)(+0.96) M-circle plus), which if real, would form a chain of near 5:3 period ratios, although the system is likely not in a mean motion resonance. Our results indicate that TOI-1266 b and c are among the lowest density sub-Neptunes around M dwarfs and likely exhibit distinct bulk compositions of a gas-enveloped terrestrial (X-env,X-b = 5.5 +/- 0.7 per cent) and a water-rich world (WMF c = 59 +/- 14 per cent), which is supported by hydrodynamic escape models. If distinct bulk compositions are confirmed through atmospheric characterization, the system's unique architecture would represent an interesting test case of inside-out sub-Neptune formation at pebble traps.