  We report the confirmation of a transiting planet around the bright, inactive
M0.5 V star TOI-1235 (TYC 4384-1735-1, V = 11.5 mag), whose transit signal was
detected in the photometric time series of Sectors 14, 20, and 21 of the TESS
space mission. We confirm the planetary nature of the transit signal, which has
a period of 3.44 d, by using precise radial velocity measurements with CARMENES
and HARPS-N spectrographs. A comparison of the properties derived for TOI-1235
b's with theoretical models reveals that the planet has a rocky composition,
with a bulk density slightly higher than Earth's. In particular, we measure a
mass of M_p = 5.9+/-0.6 M_Earth and a radius of R_p = 1.69+/-0.08 R_Earth,
which together result in a density of rho_p = 6.7+1.3-1.1 g/cm3. When compared
with other well-characterized exoplanetary systems, the particular combination
of planetary radius and mass puts our discovery in the radius gap, a transition
region between rocky planets and planets with significant atmospheric
envelopes, with few known members. While the exact location of the radius gap
for M dwarfs is still a matter of debate, our results constrain it to be
located at around 1.7 R_Earth or larger at the insolation levels received by
TOI-1235 b (~60 S_Earth), which makes it an extremely interesting object for
further studies of planet formation and atmospheric evolution.
