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Description
In the plate collection of the Harvard College Observatory, we have obtained 528 photographic magnitude estimates for the recently discovered long-period classical Cepheid ASAS 101538-5933.1 (P=51.4-days). Together with the published photoelectric and CCD observations, our data have allowed us to construct an O-C diagram spanning a time interval of 120 years. The O-C diagram has the shape of a parabola, which has made it possible to determine for the first time the quadratic light elements and to calculate the rate of evolutionary increase in the period, dP/dt=51.8(+/-4.8)s/yr or (dP/dt)/P=7.3(+/-0.7)s, in agreement with the results of theoretical calculations for the third crossing of the instability strip. The available data reduced by the method of Eddington and Plakidis reveal small random period fluctuations that do not distort the evolutionary trend in the O-C residuals.
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