PHOTOMETRY AND H-ALPHA OBSERVATIONS OF THE BINARY-SYSTEM FF AQUARII

We report on UBV photometry and moderate resolution Her spectrophotome try of the evolved binary system FF Aqr (sdOB + G8 III), performed at the Catania Astrophysical Observatory. The spectrophotometric data wer e collected in 1990 and 1991, while the photometric light curve is bas ed essentially on observations obtained in 1990. The H alpha emission line which is associated with the cool star of the system shows a clea r modulation with the orbital phase, a definite maximum occurring near the secondary eclipse. The net emission profile of all 1990 spectra, obtained by subtracting the absorption profile of a standard star, dis plays a very broad emission component (approximate to 10 Angstrom) and a narrow one (approximate to 3 Angstrom). The intensity of the latter is modulated with the orbital phase and seems to disappear near the p rimary eclipse, In the 1991 spectra the broad component is not always visible, while the narrow one is clearly present with the same behavio r as the 1990 curve. The Doppler shifts of the narrow H alpha emission , measured with respect to the photospheric lines of the cool secondar y star are consistent with an emission region placed on the G8 III sta r hemisphere facing the hotter subdwarf. The photometric light variati ons outside the eclipses show some difference with respect to that obs erved by Dworetsky et al. (1977a) and by Dorren et al, (1983). Our V l ight curve is more symmetric, the maximum light occurring at phase sim ilar or equal to 0.5, and the variation amplitude is significantly sma ller. A simple model in which the radiation from the subdwarf reaching the cool companion is completely thermalized in the atmosphere of the latter accounts for both the narrow Her emission component and UBV ou t-of eclipse light variation.