Context :

Aims : We investigated in detail the system WDS 19312+3607 , whose primary is an active M4.5Ve star previously thought to be young ( \tau \sim 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 \mu m 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 \sim 1.6 d , which we associated to orbital synchronisation .
The observed X-ray and H \alpha 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 \approx 26 pc ) , bright ( J \approx 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 .