ASYMMETRIC H-ALPHA DISK EMISSION DOMINATES THE HST FOS SPECTRUM OF CIRCINUS X-1/

Circinus X-1 is an highly variable X/radio binary source showing regul ar flux modulations with a periodicity of similar to 16.6 days, probab ly coincident with the orbital period of the binary system. Here we pr esent HST/FOS spectra of the optical counterpart taken around the pred icted X-ray maximum. Strong H alpha in emission, itself accounting for almost 50 % of the optical flux between 5000 and 6800 Angstrom, is vi sible. The H alpha profile, entirely ascribable to the accretion disk, is characterized by a remarkable asymmetry, pointing to the presence of at least two different components, i.e. a broad one at rest plus a narrower redshifted (v similar to 400 km/s) one. No other emission/abs orption line is clearly visible superimposed on the source continuum, thus making troublesome any tentative spectral classification of the c ompanion star. Furthermore, the noisy background together with a subst antial, although uncertain, interstellar reddening, makes it difficult to fit reliably any continuum spectrum at the source.