  The common envelope evolution (CEE) is vital in forming short orbital period
compact binaries. It covers many objects, such as double compact merging
binaries, type Ia supernovae progenitors, binary pulsars, and X-ray binaries.
Knowledge about the common envelope (CE) eject efficiency still needs to be
improved, though progress has been made recently. Short orbital period hot
subdwarf B star plus white dwarf binaries are the most straightforward samples
to constrain CEE physics. We apply the known orbital period-white dwarf
relation to constrain the sdB progenitor of seven sdB+WD binaries with a known
inclination angle. The average value of the CE efficiency parameter is 0.32,
which is consistent with previous studies. However, the CE efficiency might not
be a constant but is a function of the initial mass ratio based on
well-constrained sdB progenitor mass and evolutionary stage. Our results can be
used as physical inputs for binary population synthesis simulations on related
objects. A similar method can also be applied to study other short orbital
period WD binaries.
