The overwhelming majority of stellar remnants are white dwarfs. Despite their abundance and importance to, amongst others, Galactic age determinations and our understanding of type Ia supernovae fewer than a dozen white dwarfs have model-independent measurements of fundamental parameters like mass and radius. A major limitation on the observational side is that such parameters are extremely difficult to determine in a model-independant way for single white dwarfs. Close white dwarf binaries can provide these important tests. The largest class of white dwarf binaries in the Galaxy are the detached double white dwarfs, which are becoming increasingly popular as the progenitor systems of Type Ia supernovae. In recent years four eclipsing double white dwarfs have been found, creating the opportunity for precision mass and radius measurements of two white dwarfs at once. Our target, CSS 41177, contains two extremely low-gravity white dwarfs with very different temperatures, presenting us with a unique chance to test the existing mass-radius relation at its extremes. Here we propose a 2 orbit HST/COS FUV observation of CSS 41177, to accurately determine the temperature and surface gravity of the hot white dwarf. Through the flux ratio from the light curve this will at the same time constrain those of the cool white dwarf. Therefore it will allow us to add two more white dwarfs with accurate parameters to the short list of white dwarfs for which precise masses and radii are known. Note: The proposed observations are part of the doctoral thesis of Ms. Madelon C.P. Bours. The aim of the observation is to obtain a far-ultraviolet spectrum of a double-degenerate DA white dwarf. The sequence is straight-forward: target acquisition in dispersed light, followed by G140L spectroscopy using all four FP-POS positions. As this is part of Ms Bours PhD work, it would be very helpful if the observations could be scheduled early in Cycle 21. HOT STARS Default false true false false false SUPPORTED STAR DA The target was observed with GALEX, at m(fuv)=16.72 +/- 0.04 (AB) and m(nuv)=16.98 +/- 0.03 (AB). J2000 J2000 COS/FUV Spectroscopic No No G140L 1105 Phase windows are not calculated when the gap between schedulable intervals is less than 1 days, assuming no scheduling restrictions for constraint Phase 0.200/0.800. The scheduling data was generated by CASM 21.2. CASM was run in Phase II mode. This double white dwarf is eclipsing. Even though the eclipse lasts only ~minutes, we implement a phase constraint for this visit to avoid that the target acq. is done during the eclipse. FYI, a similar procedure has been successfully used in a number of visits in program 12870. (Tic-Visit :proposal-id 13421 :visit-id "01" :proposal-version -1 :trans-version "20130502" :date "03-Jul-2013 13:31:16.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "31" :si-bright-earth-limb-angle 20.0 :data-volume 330624 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 13421 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Obset :proposal-id 13421 :visit-id "01" :obset-id "A1" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 13421 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2911 :al-overhead-at-end 0 :internal-dur-at-end 442 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3244 :pcs-gap 0 ) (Tic-Orbit :proposal-id 13421 :visit-id "01" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3022 :al-overhead-at-end 0 :internal-dur-at-end 442 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3244 :pcs-gap 0 ) true 2 0