  Aims: We investigated in detail the system WDS 19312+3607, whose primary is
an active M4.5Ve star previously thought to be young (tau ~ 300-500 Ma) based
on high X-ray luminosity. Methods: We collected intermediate- and
low-resolution optical spectra taken with 2 m-class telescopes, photometric
data from the $B$ to 8 mum bands, and eleven astrometric epochs with a time
baseline of over 56 years for the two components in the system, G 125-15 and G
125-14. Results: We derived M4.5V spectral types for both stars, confirmed
their common proper motion, estimated the heliocentric distance and projected
physical separation, determined the galactocentric space velocities, and
deduced a most-probable age older than 600 Ma. We discovered that the primary,
G 125-15, is in turn an inflated, double-lined, spectroscopic binary with a
short period of photometric variability of P ~ 1.6 d, which we associated to
orbital synchronisation. The observed X-ray and Halpha emissions, photometric
variability, and abnormal radius and effective temperature of G 125-15 AB
indicate strong magnetic activity, possibly due to fast rotation. Besides, the
estimated projected physical separation between G 125-15 AB and G 125-14 of
about 1200 AU makes WDS 19312+3607 to be one of the widest systems with
intermediate M-type primaries. Conclusions: G 125-15 AB is a nearby (d ~ 26
pc), bright (J ~ 9.6 mag), active spectroscopic binary with a single
proper-motion companion of the same spectral type at a wide separation. They
are thus ideal targets for specific follow-ups to investigate wide and close
multiplicity or stellar expansion and surface cooling due to reduced convective
efficiency.
