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 \pm 0.017 h and amplitude of \Delta m = 0 \fm 0328 \pm 0 \fm 0018 , 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 \approx 1.9:1 and clearly shows that Nereid ’ s spin axis can not 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 .