SIMBAD references

Given a few determinations of the occurrence of a periodic event spanning a large interval of time, one can hope to make detailed determinations of the period and its behavior with time. This is accomplished by obtaining the correct cycle number for each observation: we show here that a modification of the Hough transform provides a robust technique for this detection. Very few observations of the event could then be all that is needed. An application of such a method to the eclipsing binary GW-Cep is given, showing that the most likely period change of this variable is opposite in sign and approximately one order of magnitude smaller than currently estimated.