  We present Hubble Space Telescope + Cosmic Origins Spectrograph (HST+COS)
data of the eclipsing double white dwarf binary CSS 41177. Due to the
temperature difference between the two white dwarfs, the HST+COS
far-ultraviolet data are dominated by the hot, primary white dwarf and allow us
to precisely measure its temperature (T1). Using eclipse observations, we also
tightly constrain the temperature of the cooler secondary white dwarf (T2). Our
results, where T1 = 22439 +/- 59 K and T2 = 10876 +/- 32 K, with the
uncertainties being purely statistical, place the secondary inside and close to
the blue edge of the empirical instability strip for low temperature
hydrogen-atmosphere white dwarfs. Dedicated high-speed photometry is encouraged
to probe for the presence of pulsations, which will constrain the border of the
instability strip as well as probe a new region of low gravity within the
strip.
