Abstract
Abstract Orbital period changes of three close binaries, TW And, TT Her and W UMi, were studied using the Kraków data base of minima of eclipsing binaries, and new photoelectric observations obtained by the authors. The interval of the observations spans roughly a century. The major orbital period change in TW And is explained by the presence of a third body with P3= 49.6 yr and a continuous secular period increase. A third body, with P3= 41.0 yr, was also found in the system TT Her. The (O–C) diagram of W UMi can be interpreted as either an abrupt orbital period decrease which occurred around the end of 1973 or as a secular orbital period decrease combined with cyclic change. In this case, the orbital period of the third body is 62.2 yr and the sum of squares of residuals is lower than for a broken-line fit, therefore cyclic variations should be preferred. In all three systems, the third body is a low-mass companion.
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