On the invisible components of some eclipsing binaries

The aim of this paper is to search for evidences for the presence of unseen companions in some eclipsing binary systems using their O-C diagrams. A total of 8507 times of minima (6890 visual and 1617 photographic or photoelectric ones) of 18 systems have been collected from the remarkable database of late Dieter Lichtenknecker (Lichtenknecker 1988) and from other more recent papers. Assuming that light-time effect (LITE) is responsible for the periodic components on the O-C diagrams sets of orbital parameters were derived using Kopal's (1978) method. In some cases (AB And, TV Gas, XX Cep, AK Her) the resulting orbital parameters and masses of the hypothetical third components are physically acceptable. Other systems show good evidence for multiplicity, but the available data sets are not suited to compute reliable solutions (W Del, U Peg, AT Peg, ST Per). For RT And, XZ And, OO Aql, Y Cam, RS CVn, CQ Cep, U CrB and MR Cyg) an unambigous identification of third components is not possible. In the cases of TW Cas and SW Lac we could not find any LITE solution at all.