  We present the confirmation of a new sub-Neptune close to the transition
between super-Earths and sub-Neptunes transiting the M2 dwarf TOI- 269 (TIC
220479565, V = 14.4 mag, J = 10.9 mag, Rstar = 0.40 Rsun, Mstar = 0.39 Msun, d
= 57 pc). The exoplanet candidate has been identified in multiple TESS sectors,
and validated with high-precision spectroscopy from HARPS and ground-based
photometric follow-up from ExTrA and LCO-CTIO. We determined mass, radius, and
bulk density of the exoplanet by jointly modeling both photometry and radial
velocities with juliet. The transiting exoplanet has an orbital period of P =
3.6977104 +- 0.0000037 days, a radius of 2.77 +- 0.12 Rearth, and a mass of 8.8
+- 1.4 Mearth. Since TOI-269 b lies among the best targets of its category for
atmospheric characterization, it would be interesting to probe the atmosphere
of this exoplanet with transmission spectroscopy in order to compare it to
other sub-Neptunes. With an eccentricity e = 0.425+0.082-0.086, TOI-269 b has
one of the highest eccentricities of the exoplanets with periods less than 10
days. The star being likely a few Gyr old, this system does not appear to be
dynamically young. We surmise TOI-269 b may have acquired its high eccentricity
as it migrated inward through planet-planet interactions.
