Discovery and mass measurement of the hot, transiting, Earth-sized planet, GJ 3929 b

We report the discovery of GJ 3929 b, a hot Earth-sized planet orbiting the nearby M3.5 V dwarf star, GJ 3929 (G 180-18, TOI-2013). Joint modelling of photometric observations from TESS sectors 24 and 25 together with 73 spectroscopic observations from CARMENES and follow-up transit observations from SAINT-EX, LCOGT, and OSN yields a planet radius of R-b = 1.150 +/- 0.040 R-circle plus, a mass of M-b = 1.21 +/- 0.42 M-circle plus, and an orbital period of P-b = 2.6162745 +/- 0.0000030 d. The resulting density of rho(b) = 4.4 +/- 1.6 g cm(-3) is compatible with the Earth's mean density of about 5.5 g cm(-3). Due to the apparent brightness of the host star (J = 8.7 mag) and its small size, GJ 3929 b is a promising target for atmospheric characterisation with the JWST. Additionally, the radial velocity data show evidence for another planet candidate with P-[c] = 14.303 +/- 0.035 d, which is likely unrelated to the stellar rotation period, P-rot = 122 +/- 13 d, which we determined from archival HATNet and ASAS-SN photometry combined with newly obtained TJO data