5. Conclusions

In this paper we have analysed, using the spectral subtraction technique, simultaneous H, Na I D₁, D₂, and He I D₃ spectroscopic observations of 18 chromospherically active binary systems.

We have found excess H in all the systems, except KT Peg which have also small emission in the Ca II H & K lines. The subtracted H profile of the more active stars of the sample (H in emission above the continuum) has very broad wings, and is well matched using a two-components Gaussian fit (narrow and broad). The broad component is primarily responsible for the observed variations of the profile, and its contribution to the total EW increases with the degree of activity. So, we have interpreted this broad component emission as arising from microflaring activity that take place in the chromosphere of these very active stars.

The H observation of the eclipsing binary system AR Lac at orbital phase 0.939, when a 0.33 fraction of the hot component is hidden by the cool component, allowed us to detect the presence of prominence-like extended material seen off the limb of the cool component through the detection of excess H absorption located at the wavelength position corresponding to the hot component. A small excess absorption in the He I D₃ line is also present at this orbital phase.

We reported the detection of an optical flare in the systems UX Ari and II Peg through the presence of the He I D₃ in emission in coincidence with the enhancement of the H emission.

We have found the He I D₃ in emission only in the two above mentioned flares, and as an absorption feature in the subtracted spectra of a large number of giant stars of the sample.

The application of the spectral subtraction technique reveals that the core of the Na I D₁ and D₂ lines are also filled-in by chromospheric emission in the more active star of the sample. The stars with only a small excess H emission do not exhibit excess emission in the Na I lines.

Acknowledgements

This work has been supported by the Universidad Complutense de Madrid and the Spanish Dirección General de Investigación Cientıfica y Técnica (DGICYT) under grant PB94-0263. We would like to thank the referee S. Catalano for suggesting several improvements and clarifications.