Fuse observations of U geminorum during outburst and decline

We have obtained far-ultraviolet (904-1187 Angstrom) spectra of U Gem in ou tburst with the Far Ultraviolet Spectroscopic Explorer. Three of the observ ations were acquired during the plateau phase of the outburst, while the fo urth was obtained during late outburst decline. The plateau spectra have co ntinuum shapes and fluxes that are approximated by steady state accretion d isk model spectra with (m) over dot similar or equal to 7 x 10(-9) M. yr(-1 ). The spectra also show numerous absorption lines of H I, He II, and 2-5 t imes ionized transitions of C, N, O, P, S, and Si. There are no emission fe atures in the spectra, with the possible exception of a weak feature on the red wing of the O VI doublet. The absorption lines are narrow (FWHM simila r to 500 km s(-1)), too narrow to arise from the disk photosphere, and at l ow velocities (less than or equal to 1700 km s(-1)). The S VI and O VI doub lets are optically thick. The absorption lines in the plateau spectra show orbital variability : in spectra obtained at orbital phases 0.53 less than or equal to Phi less than or equal to 0.79, low-ionization absorption lines appear and the central depths of the preexisting lines increase. The incre ase in line absorption occurs at the same orbital phases as previously obse rved EUV and X-ray light-curve dips. If the absorbing material is in (near- ) Keplerian rotation around the disk, it must be located at large disk radi i. The final observation occurred when U Gem was about 2 mag from optical q uiescence. The spectra are dominated by emission from an similar or equal t o 43,000 K, metal-enriched white dwarf (WD). The inferred radius of the WD is 4.95 x 10(8) cm, close to that observed in quiescence. Allowing for a ho t heated region on the surface of the WD improves the fit to the spectrum a t short (less than 960 Angstrom) wavelengths.