2002A&A...392..895B

Result. of a new light-curve analysis and period variation study of the eclipsing binary IM Aur are presented. Four solutions are given according to the different spectral types of the primary, available in the literature. Multicolour photometry and the period analysis both indicate a close (≃0.1") third physical member. If it is considered as a main-sequence star, then its spectral type is A2V-A8V, with a mass of m₃≃1.7-2.5M_☉. The light-time effect is modelled by the authors' simultaneous secular and periodic terms fitting code. Clear evidence of a secular period change is also revealed. The detailed effects of the perturbations by a third member of a close binary system are extensively studied, using one of the authors' latest numerical integrator code. Although the amplitudes of the expected O-C changes are too small and also the data set is far undersampled for clear confirmation of the theoretical expectation of the third body-perturbed eclipsing binary O-C curve, some typical patterns are shown for their possible identification amongst the O-C curves of other closer systems (like Algol, lambda Tau) in the near future.