Luminosity and colour variations of 88 HER through phase changes fromthe far UV to the visual region. I. A confrontation with traditional Be-star modelling.

In this paper, hereafter called Paper I, we present and analyze the luminosity and colour variations of 88 Her in the wavelength range 1550 Å-55OO Å, during the period 1972-1983 where the star has exhibited two different phases: a quasi-normal B phase and a new Be-shell phase of increasing strength. The observations show that: (i) The star is most luminous in all the studied wavelengths during the quiescent quasi-normal B phase. It is faintest during the early stages of the Be-shell (Hydrogen + metals) phase. (ii) The total luminosity of the star in the 1550-5500 Å range drops by about one third of its value, from the quasi-normal B phase to the Be-shell phase. (iii) The total luminosity increases by 10 % from 1979 to 1983 in all the observed wavelength-range when the strength of the Be-shell spectrum increases (iv) There is not one single spectral region in the 1550- 5500 Å range, where the brightness of the star does not change. The flux variations follow the same trend in the far UV and in the visual regions, but the largest amplitudes of variations are exhibited in the far UV. We discuss the observed variations of the radiative flux in the framework of traditional Be-star modelling and conclude that model predictions are contradicted by the two observations: (a) Minimum luminosity is not observed at maximum development of the shell spectrum; on the contrary, it occurs when the shell spectrum is still very mild. (b) The development of the shell spectrum is not accompanied by a decrease in luminosity; on the contrary, the luminosity increases in all the investigated wavelengths.