  A finite sequence of equidistant samples (a sample train) of a periodic
signal can be identified with a point in a multi-dimensional space. Such a
point depends on the sampled signal, the sampling period, and the starting time
of the sequence. If the starting time varies, then the corresponding point
moves along a closed curve. We prove that such a curve, i.e., the set of all
sample trains of a given length, determines the period of the sampled signal,
provided that the sampling period is known. This is true even if the trains are
short, and if the samples comprising trains are taken at a sub-Nyquist rate.
The presented result is proved with a help of the theory of rotation numbers
developed by Poincar\'e. We also prove that the curve of sample trains
determines the sampled signal up to a time shift, provided that the ratio of
the sampling period to the period of the signal is irrational. Eventually, we
give an example which shows that the assumption on incommensurability of the
periods cannot be dropped.
