  In this paper we present an analysis of Kepler K2 mission Campaign 3
observations of the irregular Neptune satellite, Nereid. We determined a
rotation period of P=11.594(+/-)0.017 h and amplitude of dm=0.0328(+/-)00018,
confirming previous short rotation periods obtained in ground based
observations. The similarities of light curve amplitudes between 2001 and 2015
show that Nereid is in a low-amplitude rotation state nowadays and it could
have been in a high-amplitude rotation state in the mid 1960's. Another
high-amplitude period is expected in about 30 years. Based on the light curve
amplitudes observed in the last 15 years we could constrain the shape of Nereid
and obtained a maximum a:c axis ratio of 1.3:1. This excludes the previously
suggested very elongated shape of a:c=1.9:1 and clearly shows that Nereid's
spin axis cannot be in forced precession due to tidal forces. Thermal emission
data from the Spitzer Space Telescope and the Herschel Space Observatory
indicate that Nereid's shape is actually close to the a:c axis ratio limit of
1.3:1 we obtained, and it has a very rough, highly cratered surface
