Ultraviolet observations of HD 77581 (= 2U 0900-40).

The absolute flux distribution in the spectral range from 1350 A to 2740 A of HD 77581 is presented here. The companion of this star is the X-ray source 2U 0900-40. The ultraviolet spectrum has been obtained with the S2/68 Ultraviolet Sky Survey telescope in the TDI satellite. Its flux distribution corrected for interstellar reddening determined from the observed ultraviolet colour (m_2910 - m_2500), is found to be similar to that of a B0 supergiant star, epsilon Ori (HD 37128).