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ASTRONOMICAL JOURNAL, vol.114, no.6, pp.2753-2763, 1997 (SCI-Expanded)
18 new times of minimum light for YY Eri were determined from relatively new or unpublished photoelectric observations from Korea, New Zealand, and Turkey, and added to a collection of all other times of minima available to us. These data were then intensively analyzed, by reference 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 superposed 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 underlying sinusoidal variations, rather than real sudden changes of period, associated with sudden mass ejections in the system. An upward parabolic O-C diagram corresponds to a secular period increase, usually interpreted as mass transfer from the less to the more massive component. The sinusoidal character suggests an unseen third body, or cyclic strong magnetic activity on at least one of the component stars. The long term sinusoidal light level variation is supported by cyclical magnetic 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].