Context : We have performed observations of water maser emission towards a sample of low-mass protostars , in order to investigate the properties of jets associated with the earliest stages of star formation and their interaction with the surrounding medium .

Aims : The main aim is to measure the absolute positions and proper motions of the H _ { 2 } O spots in order to investigate the kinematics of the region from where the jet is launched .

Methods : We imaged the protostars in the nearby region NGC 1333–IRAS 4 in the water maser line at 22.2 GHz by using the VLBA in phase-reference mode at the milliarcsecond scale over four epochs , spaced by one month to measure proper motions .

Results : Two protostars ( A2 and B ) were detected in a highly variable H _ { 2 } O maser emission , with an active phase shorter than four weeks .
The H _ { 2 } O maps allow us to trace the fast jet driven by the B protostar : we observed both the red- and blue-shifted lobes very close to the protostar , \leq 35 AU , moving away with projected velocities of \sim 10–50 km s ^ { -1 } .
The comparison with the molecular outflow observed at larger scale suggests a jet precession with a 18 \arcmin yr ^ { -1 } rate .
By measuring the positional spread of the H _ { 2 } O spots we estimate a jet width of \sim 2 AU at a distance of \sim 12 AU from the driving protostar .

Conclusions :