MG-II H-AND-K LINE DIAGNOSTICS OF IM-PEGASI

We have analysed the Mg II h&k resonance lines of IM Peg (KII-III) on 14 high resolution IUE LWR spectra and compared the results coming fro m visual spectroscopic and UBV(RI)(C) photometric data. The 'clean' sp ectra (free from interstellar absorption) were modelled using a convol ution of the rotational profile with a Gaussian profile, that represen ts macroturbulence and broadenings of other origin, the former dominat ing. Significant variations in the h&k line fluxes and widths were fou nd throughout the orbital phase range. A line broadening effect is fou nd at maximum spot visibility both in the k and h lines. The Mg II lin e widths increase linearly with the flux of the transition region line s as a function of rotational phase. Anticorrelations as a function of phase were found between the Mg II line fluxes and the higher tempera ture lines. The k line is broader than the h line, correlated with a s lope greater than unity, in keeping with a greater radiation transfer effect in the k line, and a non-optically thin medium at all phases. W e observe a systematic asymmetry in the ratio of the h to k line profi les at all phases. The spectral diagnostics thus suggest the appearanc e of a plage region on the stellar disc. We obtained new orbital eleme nts of the IM Peg system, deriving a small, but non-zero eccentricity.