Period change in Magellanic Cloud Cepheids

The data for period change in some 115 Cepheid variable stars in the Magellanic Clouds are presented. The accuracy of each determination is estimated and for all but three of the stars it is possible to estimate whether the rate of change of period prior to 1966/67 is maintained subsequently. About 40 per cent of the stars show period variations, similar to the percentage estimated for galactic Cepheids by Hoffmeister (1967). About half of these variations cannot be regarded as being constant in rate. While evolutionary changes in the energy-generating processes may be the cause of some of the secular trends, the irregularities preclude these as being the sole cause; also, light-time effects in binary orbits would constitute a very unlikely cause. Probably the interpretation of the data lies either in small atmospheric changes that produce phase-discontinuities that accumulate or in small luminosity changes that produce random period changes. A stochastic test should differentiate these possible causes if suitable data were available.