  We report here on the photometric and kinematic properties of a well defined
group of nearly 200 low-mass pre-main sequence stars, concentrated within ~ 1
deg of the early-B star 25 Ori, in the Orion OB1a sub-association. We refer to
this stellar aggregate as the 25 Orionis group. The group also harbors the
Herbig Ae/Be star V346 Ori and a dozen other early type stars with photometry,
parallaxes, and some with IR excess emission, consistent with group membership.
The number of high and low-mass stars is in agreement with expectations from a
standard Initial Mass Function. The velocity distribution for the young stars
in 25 Ori shows a narrow peak centered at 19.7 km/s, very close to the velocity
of the star 25 Ori. Our results provide new and compelling evidence that the 25
Ori group is a distinct kinematic entity, and that considerable space and
velocity structure is present in the Ori OB1a sub-association. The low-mass
members follow a well defined band in the color-magnitude diagram, consistent
with an isochronal age of ~ 7-10 Myr, depending on the assumed evolutionary
model. The highest density of members is located near the star 25 Ori, but the
actual extent of the cluster cannot be well constrained with our present data.
In a simple-minded kinematic evolution scenario, the 25 Ori group may represent
the evolved counterpart of a younger aggregate like the sigma Ori cluster. The
25 Ori stellar aggregate is the most populous ~ 10 Myr sample yet known within
500 pc, setting it as an excellent laboratory to study the evolution of
solar-like stars and protoplanetary disks.
