A sub-Neptune planet around TOI-1695 discovered and characterized with SPIRou and TESS

TOI-1695 is a V-mag=13 M-dwarf star from the northern hemisphere at 45\(\,\)pc from the Sun, around which a 3.134-day periodic transit signal from a super-Earth candidate was identified in TESS photometry. With a transit depth of 1.3\(\,\)mmag, the radius of candidate TOI-1695.01 was estimated by the TESS pipeline to be 1.82\(\,\)R\(_\oplus\) with an equilibrium temperature of \(\sim 620\,\)K. We successfully detect a reflex motion of the star and establish it is due to a planetary companion at an orbital period consistent with the photometric transit period thanks to a year-long radial-velocity monitoring of TOI-1695 by the SPIRou infrared spectropolarimeter. We use and compare different methods to reduce and analyse those data. We report a 5.5-\(\sigma\) detection of the planetary signal, giving a mass of \(5.5 \pm 1.0\,\)M\(_\oplus\) and a radius of \(2.03 \pm 0.18\,\)R\(_\oplus\). We derive a mean equilibrium planet temperature of \(590 \pm 90\,\)K. The mean density of this small planet of \(3.6 \pm 1.1\,\)g\(\,\)cm\(^{-3}\) is similar (1.7-\(\sigma\) lower) than that of the Earth. It leads to a non-negligible fraction of volatiles in its atmosphere with \(f_{H,He}=0.28^{+0.46}_{-0.23}\)% or \(f_\text{water}=23 \pm 12\)%. TOI-1695\(\,\)b is a new sub-Neptune planet at the border of the M-dwarf radius valley that can help test formation scenarios for super-Earth/sub-Neptune-like planets.