5 Discussion and conclusions

Several very interesting phenomena are present in the light curves of V1162 Ori, including multiperiodicity and cyclic amplitude variability. The O-C analysis reveals the presence of a linear period change whose size is too large to be reconciled with evolutionary changes as given by Breger & Pamyatnykh (1998). However, these authors have collected available information on observed period changes in $\delta$ Scuti stars and find that the observed values, which are distributed nearly evenly between increasing and decreasing periods, disagree with stellar evolution calculations. They conclude that the observed linear period changes are not caused by evolutionary effects, but rather by long-period binarity or nonlinear mode interactions.

On top of the linear change are O-C variations which can be explained by frequent period changes or a cyclic variation combined with sudden changes. In both cases is the time scale of the period variation similar to that of the amplitude modulation, making a common origin probable.

Cyclic variations in the O-C diagram are explained either by the light time effect caused by the motion of the pulsating star in a binary system, or by beating of two (or more) very closely-spaced frequencies. These possibilities were put forward for V1162 Ori already in PaperI. In Sect. 3.2.2 we noticed a peak in the amplitude spectrum very close to f₁, but considering the nearly cyclic amplitude and phase variations present in the data, such a close peak would always be expected, as discussed in Sect. 3.3.1.

Including this close peak in the frequency solution still leads to an increased noise level in the residual amplitude spectrum of 15% even if we take the linear variation in period of f₁ into account. Furthermore, with a pure beat of f₁ with a single close frequency, we cannot explain the deviations from a sinusoidal shape in the O-C diagram corrected for the linear period change (Fig. 14, middle and lower panels). A linear change combined with beating between two close frequencies is therefore, based on the present data, not a likely (or at least not the only) explanation of the observed changes.

In the case of binarity there is more room for deviations from strict periodicity. We get from the light time effect (following Irwin (1959) and assuming a circular orbit), that a binary configuration realistically models the possible cyclic part of the O-C variations. The separation between the two components would be small, of the order of 1 AU. We have no good interpretation of the amplitude variations in case of binarity: as f₁ is probably a radial mode, a changing aspect angle would not give rise to amplitude variations. One could speculate that in such a relatively close system the amplitude variations could be caused by tidal deformation of V1162 Ori, but in this case one would expect all the detected frequencies to behave in a similar way, which is not the case. We note that such a binary system is not expected to give rise to detectable low-frequency variability in the light curves, even for a mass ratio of 1.

In conclusion, we have shown that V1162 Ori, previously considered monoperiodic, is a multiperiodic $\delta$ Scuti star. Apart from the main mode f₁ (with harmonics), we find 5 additional frequencies in the amplitude spectrum. All these have very low amplitude and are most likely non-radial. V1162 Ori displays period and (cyclic) amplitude variability. The main pulsational period shows a linear period change with a change rate of $(1/P)({\rm d}p/{\rm d}t) \sim$ $-2.4\times10^{-5}$ yr^-1. In addition, the O-C diagram reveals the presence of additional changes on the same time scale as the amplitude modulation.

The goals of the present campaign have partly been reached: we have obtained a much better knowledge of the changes present in the light curves of V1162 Ori - but we have not been able to determine their cause. Further photometric observations are clearly needed, and so is an extensive spectroscopic investigation spanning several years to establish or reject a possible binary nature of this star.

Acknowledgements

TA and CS acknowledge financial support from the Belgian Fund for Scientific Research (FWO). This project was supported by the Flemish Ministry for Foreign Policy, European Affairs, Science and Technology, under contract BIL 98/11/52, and the National Research Foundation of South Africa. LMF acknowledges support from IUAP P4/05 financed by Belgian DWTC/SSTC. AB acknowledges support through a grant of the Conselho Nacional de Desenvolvimento Científico e Tecnológico (grant No. 301029). PVC is grateful to the Royal Observatory of Belgium for putting at his disposal material acquired by project G.0265.97 of the Fund for Scientific Research (FWO) - Flanders (Belgium); sincere thanks go to Dr. J. Cuypers. Our research has made use of the SIMBAD database operated at CDS, Strasbourg, France, and the NASA Astrophysics Data System.