HST+COS spectra of the double white dwarf CSS 41177 place the secondary inside the pulsational instability strip

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 = 22 439 ± 59 K and T2 = 10 876 ± 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.