  We have performed 5 night spectroscopic observation of the Halpha line of EV
Lac with a medium wavelength resolution (R~ 10,000) using the 2m Nayuta
telescope at the Nishi-Harima Astronomical Observatory. EV Lac always possesses
the Halpha emission line; however, its intensity was stronger on August 15,
2015 than during other four-night periods. On this night, we observed a rapid
rise (~ 20min) and a subsequent slow decrease (~ 1.5h) of the emission-line
intensity of Halpha, which was probably caused by a flare. We also found an
asymmetrical change in the Halpha line on the same night. The enhancement has
been observed in the blue wing of the Ha line during each phase of this flare
(from the flare start to the flare end), and absorption components were present
in its red wing during the early and later phases of the flare. Such blue
enhancement (blue asymmetry) of the Halpha line is sometimes seen during solar
flares, but only during the early phases. Even for solar flares, little is
known about the origin of the blue asymmetry. Compared with solar-flare models,
the presented results can lead to the understanding of the dynamics of stellar
flares.
