Chandra Low Energy Transmission Grating Spectrograph observations of the pre-cataclysmic binary V471 Tau have been used to estimate the C/N abundance ratio of the K dwarf component for the first time .
While the white dwarf component dominates the spectrum longward of 50 Å , at shorter wavelengths the observed X-ray emission is entirely due to coronal emission from the K dwarf .
The H-like 2p ^ { 2 } P _ { 3 / 2 , 1 / 2 } \rightarrow 1s ^ { 2 } S _ { 1 / 2 } resonance lines of C and N yield an estimate of their logarithmic abundance ratio relative to the Sun of [ C/N ] = -0.38 \pm 0.15 —half of the currently accepted solar value .
We interpret this result as the first clear observational evidence for the presumed common envelope phase of this system , during which the surface of the K dwarf was contaminated by CN-cycle processed material dredged up into the red giant envelope .
We use the measured C/N ratio to deduce that 0.015- 0.04 M _ { \odot } was accreted by the K dwarf while engulfed , and show that this is consistent with a recent tentative detection of ^ { 13 } C in the K dwarf photosphere , and with the measured Li abundance in the scenario where the red giant companion was Li-rich during the common envelope phase .