V471 Tauri , an eclipsing system consisting of a hot DA white dwarf ( WD ) and a dK2 companion in a 12.5-hour orbit , is the prototype of the pre-cataclysmic binaries .
The late-type component is magnetically active , due to its being constrained to rotate synchronously with the short orbital period .
During a program of ultraviolet spectroscopy of V471 Tau , carried out with the Goddard High Resolution Spectrograph ( GHRS ) onboard the Hubble Space Telescope , we serendipitously detected two episodes in which transient absorptions in the Si iii 1206 Å resonance line appeared suddenly , on a timescale of \lesssim 2 min .
The observations were taken in a narrow spectral region around Lyman- \alpha , and were all obtained near the two quadratures of the binary orbit , i.e. , at maximum projected separation ( \sim 3.3 R _ { \sun } ) of the WD and K star .

We suggest that these transient features arise when coronal mass ejections ( CME ’ s ) from the K2 dwarf pass across the line of sight to the WD .
Estimates of the velocities , densities , and masses of the events in V471 Tau are generally consistent with the properties of solar CME ’ s .
Given our detection of 2 events during 6.8 hr of GHRS observing , along with a consideration of the restricted range of latitudes and longitudes on the K star ’ s surface that can give rise to trajectories passing in front of the WD as seen from Earth , we estimate that the active V471 Tau dK star emits some 100–500 CME ’ s per day , as compared to \sim 1–3 per day for the Sun .
The K dwarf ’ s mass-loss rate associated with CME ’ s is at least ( 5– 25 ) \times 10 ^ { -14 } M _ { \odot } yr ^ { -1 } , but it may well be orders of magnitude higher if most of the silicon is in ionization states other than Si iii .