  We observed comet C/2012 S1 (ISON) during six nights in February 2013 when it
was at 4.8 AU from the sun. At this distance and time the comet was not very
active and it was theoretically possible to detect photometric variations
likely due to the rotation of the cometary nucleus. The goal of this work is to
obtain differential photometry of the comet inner coma using different aperture
radii in order to derive a possible rotational period. Large field of view
images were obtained with a 4k x 4k CCD at the f/3 0.77m telescope of La Hita
Observatory in Spain. Aperture photometry was performed in order to get
relative magnitude variation versus time. Using calibrated star fields we also
obtained ISON's R-magnitudes versus time. We applied a Lomb-Scargle periodogram
analysis to get possible periodicities for the observed brightness variations,
directly related with the rotation of the cometary nucleus. The comet light
curve obtained is very shallow, with a peak-to-peak amplitude of 0.03 $\pm$
0.02 mag. A tentative synodic rotational period (single-peaked) of 14.4 $\pm$
1.2 hours for ISON's nucleus is obtained from our analysis, but there are other
possibilities. We studied the possible effect of the seeing variations in the
obtained periodicities during the same night, and from night to night. These
seeing variations had no effect on the derived periodicity. We discuss and
interpret all possible solutions for the rotational period of ISON's nucleus.
