The long-period semi-detached giant binary system 5 Ceti.

The binary system 5 Ceti consists of a K3 III star at or near its Roche lobe and a small hot companion best studied in the satellite ultraviolet. The hot radiation source at the secondary star is immersed in the chromosphere of the K giant, and the slight enhancements of a few UV absorption lines at secondary minimum are used to estimate chromospheric column densities. The variability and flatness of the spectrum is evidence for considering the object a main-sequence star accreting mass by way of Roche-lobe overflow at the rate of 5 X 10 to the minus 7 solar masses/yr. Thus 5 Ceti may represent the elusive rapid stage of mass transfer in which matter is lost from the shrinking Roche lobe of the more massive star. The Fe II UV spectrum in interacting binaries is also discussed.