We have used the Goddard High Resolution Spectrograph onboard the Hubble Space Telescope to obtain Lyman- \alpha spectra of the hot white-dwarf ( WD ) component of the short-period eclipsing DA+dK2 pre-cataclysmic binary V471 Tauri , a member of the Hyades star cluster .

Radial velocities of the WD were determined from eight post-COSTAR spectra , obtained near the two quadratures of the orbit .
When combined with ground-based measurements of the dK velocities , eclipse timings , and a determination of the dK star ’ s rotational velocity , the data constrain the orbital inclination to be i = 77 ^ { \circ } , and yield dynamical masses for the components of M _ { WD } = 0.84 and M _ { dK } = 0.93 M _ { \sun } .
Model-atmosphere fitting of the Ly \alpha profile provides the effective temperature ( 34,500 K ) and surface gravity ( \log g = 8.3 ) of the WD .

The radius of the dK component is about 18 % larger than that of a normal Hyades dwarf of the same mass .
This expansion is attributed to the large degree of coverage of the stellar surface by starspots , which is indicated both by radiometric measurements and ground-based Doppler imaging ; in response , the star has expanded in order to maintain the luminosity of a 0.93 M _ { \sun } dwarf .

The radius of the WD , determined from a radiometric analysis and from eclipse ingress timings , is 0.0107 R _ { \sun } .
The position of the star in the mass-radius plane is in full accord with theoretical predictions for a degenerate carbon-oxygen WD with a surface temperature equal to that observed .
The position of the WD in the H-R diagram is also fully consistent with that expected for a WD with our dynamically measured mass .
Both comparisons with theory are probably the most stringent yet made for any WD .
The theoretical cooling age of the WD is 10 ^ { 7 } yr .

The high effective temperature and high mass of the WD present an evolutionary paradox .
The WD is the most massive one known in the Hyades , but also the hottest and youngest , in direct conflict with expectation .
We examine possible resolutions of the paradox , including the possibility of a nova outburst in the recent past , but conclude that the most likely explanation is that the WD is indeed very young , and is descended from a blue straggler .
A plausible scenario is that the progenitor system was a triple , with a close inner pair of main-sequence stars whose masses were both similar to that of the present cluster turnoff .
These stars became an Algol-type binary , which merged after several hundred million years to produce a single blue straggler of about twice the turnoff mass .
When this star evolved to the AGB phase , it underwent a common-envelope interaction with a distant dK companion , which spiralled down to its present separation , and ejected the envelope .
We estimate that the common-envelope efficiency parameter , \alpha _ { CE } , was of order 0.3–1.0 , in good agreement with recent hydrodynamical simulations .