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The two-planet transiting system LHS 1140 has been extensively observed sinceits discovery in 2017, notably with $Spitzer$, HST, TESS, and ESPRESSO, placingstrong constraints on the parameters of the M4.5 host star and its smalltemperate exoplanets, LHS 1140 b and c. Here, we reanalyse the ESPRESSOobservations of LHS 1140 with the novel line-by-line framework designed tofully exploit the radial velocity content of a stellar spectrum while beingresilient to outlier measurements. The improved radial velocities, combinedwith updated stellar parameters, consolidate our knowledge on the mass of LHS1140 b (5.60$\pm$0.19 M$_{\oplus}$) and LHS 1140 c (1.91$\pm$0.06 M$_{\oplus}$)with unprecedented precision of 3%. Transits from $Spitzer$, HST, and TESS arejointly analysed for the first time, allowing us to refine the planetary radiiof b (1.730$\pm$0.025 R$_{\oplus}$) and c (1.272$\pm$0.026 R$_{\oplus}$).Stellar abundance measurements of refractory elements (Fe, Mg and Si) obtainedwith NIRPS are used to constrain the internal structure of LHS 1140 b. Thisplanet is unlikely to be a rocky super-Earth as previously reported, but rathera mini-Neptune with a $\sim$0.1% H/He envelope by mass or a water world with awater-mass fraction between 9 and 19% depending on the atmospheric compositionand relative abundance of Fe and Mg. While the mini-Neptune case would not behabitable, a water-abundant LHS 1140 b potentially has habitable surfaceconditions according to 3D global climate models, suggesting liquid water atthe substellar point for atmospheres with relatively low CO$_2$ concentration,from Earth-like to a few bars.

New Mass and Radius Constraints on the LHS 1140 Planets -- LHS 1140 b is Either a Temperate Mini-Neptune or a Water World