6 Conclusions

Our study of the star HD 131120 was based on multicolour photometry and high-resolution spectroscopy spread over respectively one and two years. From the period analysis on all the data it is clear that this star is monoperiodic with a period of 1.569 days. We compared the variations in the data with a non-radial pulsation model. The mode identification by three methods does not allow to choose one particular pulsation mode. Moreover, the line-profile behaviour of the Si and He lines is very different as far as the temporal behaviour and phasing is concerned. Such a difference cannot be explained by a pulsational model.

Next we tried to interpret the monoperiodic variations by rotational modulation. We compared the line-profile variations with a model with two spots in order to reproduce the first moment of the observed spectral lines. This leads to very good results for both He and Si lines. Moreover, the best spot model is able to reproduce the behaviour of the first three moments of the lines, as well as the large equivalent width variations. Moreover, we showed that a spot model can well explain the different temporal behaviour of the moments of the two different lines. This work results in a model for a He-underabundant and Si-overabundant star, which is fully compatible with results reported in the literature. We conclude that rotational modulation is the best explanation of the variability of HD 131120 and remove the star from the list of SPBs. From the estimated i, $v_{\Omega }$ and frequency we derive a stellar radius of 3.4 $R_{\odot}$, which is compatible with the value derived from photometry.

In the sample of selected SPBs, three other stars have moments that vary in a similar way to those of HD 131120. Two of these stars were not reported before as Bp stars. We plan to test the rotational modulation model for these stars as well in order to try to explain their variability in full detail. The outcome of this study will be given in a forthcoming paper.

With this study we provide a very convenient way to discriminate between stellar pulsation and stellar spots. Indeed, the moment variations of the line profiles of different spectral lines have the same behaviour, as far as amplitudes and phases are concerned, when one is dealing with a pulsating star. For spotted stars, the moments of the line profiles clearly differ for different lines. Additionally, we have shown that the moment variations of the spectral lines of spotted stars allow one to derive very accurately the geometry and other physical characteristics of the surface spots.

Acknowledgements

We thank the referee, Dr. R. J. Dukes Jr., for his useful suggestions, which helped us to improve this paper. We used the SIMBAD astronomical database operated at the CDS in Strasbourg, France. This work was supported by the P. A. I. (Pôle d'Attraction Interuniversitaire).