  Despite their activity, low-mass stars are of particular importance for the
search of exoplanets by the means of Doppler spectroscopy, as planets with
lower masses become detectable. We report on the discovery of a planetary
companion around HD 180617, a bright J = 5.58 mag, low-mass M = 0.45 M_{sun}
star of spectral type M2.5 V. The star, located at a distance of 5.9 pc, is the
primary of the high proper motion binary system containing vB 10, a star with
one of the lowest masses known in most of the twentieth century. Our analysis
is based on new radial velocity (RV) measurements made at red-optical
wavelengths provided by the high-precision spectrograph CARMENES, which was
designed to carry out a survey for Earth-like planets around M dwarfs. The
available CARMENES data are augmented by archival Doppler measurements from
HIRES and HARPS. Altogether, the RVs span more than 16 years. The modeling of
the RV variations, with a semi-amplitude of K = 2.85-0.25/+0.16m/s yields a
Neptune-like planet with a minimum mass of 12.2-1.4/+1.0 M_{Earth} on a
105.90-0.10/+0.09d circumprimary orbit, which is partly located in the host
star's habitable zone. The analysis of time series of common activity
indicators does not show any dependence on the detected RV signal. The
discovery of HD 180617 b not only adds information to a currently hardly filled
region of the mass-period diagram of exoplanets around M dwarfs, but the
investigated system becomes the third known binary consisting of M dwarfs and
hosting an exoplanet in an S-type configuration. Its proximity makes it an
attractive candidate for future studies.
