Discovery of new members of the nearby young stellar association in
  Cepheus
  Young field stars are hardly distinguishable from older ones because their
space motion rapidly mixes them with the stellar population of the Galactic
plane. Nevertheless, a careful target selection allows for young stars to be
spotted throughout the sky.
  We aim to identify additional sources associated with the four young comoving
stars that we discovered towards the CO Cepheus void and to provide a
comprehensive view of the Cepheus association.
  Based on multivariate analysis methods, we have built an extended sample of
193 young star candidates, which are the optical and infrared counterparts of
ROSAT All-Sky Survey and XMM-Newton X-ray sources. From optical spectroscopic
observations, we measured their radial velocity with the cross-correlation
technique. We derived their atmospheric parameters and projected rotational
velocity with the code ROTFIT. We applied the subtraction of inactive templates
to measure the lithium equivalent width, from which we infer their lithium
abundance and age. Finally, we studied their kinematics using the second Gaia
data release.
  Our sample is mainly composed of young or active stars and multiple systems.
We identify two distinct populations of young stars that are spatially and
kinematically separated. Those with an age between 100 and 300 Myr are mostly
projected towards the Galactic plane. In contrast, 23 of the 37 sources younger
than 30 Myr are located in the CO Cepheus void, and 21 of them belong to the
stellar kinematic group that we previously reported in this sky area. We report
a total of 32 bona fide members and nine candidates for this nearby (distance =
157$\pm$10 pc) young (age = 10-20 Myr) stellar association. According to the
spatial distribution of its members, the original cluster is already dispersed
and partially mixed with the local population of the Galactic plane.
