Period changes, evolution, and multiperiodicity in the peculiar populationII Cepheid RU Camelopardalis

We have studied the period change in the peculiar Population II Cepheid RU Cam, which abruptly decreased in amplitude in 1965-1966. The O-C diagram pr ior to 1965-1966 can best be explained as the superposition of small, rando m, cycle-to-cycle changes in period, plus a constant linear decrease whose timescale (31,000 yr) is consistent with evolutionary predictions. The O-C diagram after 1965-1966 can best be explained as the superposition of much larger random, cycle-to-cycle changes (which mask the evolutionary changes) , plus wavelike changes in O-C on a timescale of 10-20 periods. These-and t he variations in the amplitude of the star that occur on the same timescale -may be due to multiperiodicity.