An Orbital Period Change for the Intermediate Polar XY Ari

Orbital period changes in close binaries are poorly understood, the only secure fact being that they are incompatible with the trends expected in conventional evolutionary models. Instead they might indicate the effects of irradiation on the secondary star, or of solar- type magnetic cycles in the secondary star. XY Ari is the only intermediate polar showing total X-ray eclipses, and is thus one of the few systems which can be used to detect orbital period changes with confidence. The observations requested here, combined with archival Ginga, ASCA and XTE data, will be sufficient to detect period changes of the magnitude seen in other close binaries.