THE PERIOD CHANGES OF YY ERIDANI

18 new times of minimum light for YY Eri were determined from relative ly new or unpublished photoelectric observations from Korea, New Zeala nd, and Turkey, and added to a collection of all other times of minima available to us. These data were then intensively analyzed, by refere nce to an O-C diagram, to deduce the general form of period variation for YY Eri. The O-C diagram could be either of quasi-sinusoidal form s uperposed on an upward parabola, or a set of abrupt changes. If abrupt changes, they appear to have occurred alternately in a pattern of two increases following one decrease. This may be an indication of underl ying sinusoidal variations, rather than real sudden changes of period, associated with sudden mass ejections in the system. An upward parabo lic O-C diagram corresponds to a secular period increase, usually inte rpreted as mass transfer from the less to the more massive component. The sinusoidal character suggests an unseen third body, or cyclic stro ng magnetic activity on at least one of the component stars. The long term sinusoidal light level variation is supported by cyclical magneti c activity effects in the orbital period. However, the existence of a third body cannot be ruled out by the present data. (C) 1997 American Astronomical Society. [S0004-6256(97)01512-4].