  We report the detection of a long-duration optical flare in the recenltly
discovered, X-ray selected, chromospherically binary 2RE J0743+224. The high
resolution echelle spectroscopic observations taken in 12-21th January 1998
exhibit a dramatic increase in the chromospheric emissions (H_alpha and CaII
IRT lines) that we interpret as a flare based on: the temporal evolution of the
event, the broad component observed in the H_alpha line profile, the detection
of the HeI D_3 in emission and a filled-in HeI 6678 A. During these obsevations
we detect a LiI 6708 A line enhancement which is clearly related with the
temporal evolution of the flare. The maximum LiI enhancement occurs just after
the maximum chromospheric emission observed in the flare. A significant
increase of the 6Li/7Li isotopic ratio is also detected. From all this we
suggest that this LiI enhancement is produced by spallation reactions during
the flare. This is the first time that such LiI enhancement associate with a
stellar flare is reported, and probably the long-duration of this flare is a
key factor for this detection. A large fraction of the stellar surface seems to
be covered by starspots during the event, as we deduce for the analysis of the
TiO 7055 A band. Thus taking into account that LiI line is very temperature
sensitive, we can not discard that the LiI variations are related the presence
of starspots. However, the correlation with the temporal evolution of the
flare, lack of detection of changes in the other photospheric absorption lines,
and the large changes observed in the core of the LiI, as predict the models,
argue in favour of the hypothesis that the LiI is produced during the flare.
