  We report the first $BV$ light curves and high-resolution spectra of the
post-mass transfer binary star WASP 0131+28 to study the absolute properties of
extremely low-mass white dwarfs. From the observed spectra, the double-lined
radial velocities were derived, and the effective temperature and rotational
velocity of the brighter, more massive primary were found to be $T_{\rm eff,1}
= 10,000 \pm 200$ K and $v_1\sin$$i$ = 55 $\pm$ 10 km s$^{-1}$, respectively.
The combined analysis of the {\it TESS} archive data and ours yielded the
accurate fundamental parameters of the program target. The masses were derived
to about 1.0 \% accuracy and the radii to 0.6 \%, or better. The secondary
component's parameters of $M_2 = 0.200 \pm 0.002$ M$_\odot$, $R_2 = 0.528 \pm
0.003$ R$_\odot$, $T_{\rm eff,2}$ = 11,186 $\pm$ 235 K, and $L_2 = 3.9 \pm 0.3$
L$_\odot$ are in excellent agreement with the evolutionary sequence for a
helium-core white dwarf of mass 0.203 M$_\odot$, and indicates that this star
is halfway through the constant luminosity phase. The results presented in this
article demonstrate that WASP 0131+28 is an EL CVn eclipsing binary in a thin
disk, which is formed from the stable Roche-lobe overflow channel and composed
of a main-sequence dwarf with a spectral type A0 and a pre-He white dwarf.
