  Most subdwarf B (sdB) + Helium white dwarf (He WD) binaries are believed to
be formed from a particular channel. In this channel, the He WDs are produced
first from red giants (RGs) with degenerate cores via stable mass transfer and
sdB stars are produced from RGs with degenerate cores via common envelope (CE)
ejection. They are important for the studies of CE evolution, binary evolution,
and binary population synthesis. However, the relation between WD mass and
orbital period of sdB + He WD binaries has not been specifically studied. In
this paper, we first use a semi-analytic method to follow their formation and
find a WD mass and orbital period relation. Then we use a detailed stellar
evolution code to model their formation from main-sequence binaries. We find a
similar relation between the WD mass and orbital period, which is in broad
agreement with observations. For most sdB + He WD systems, if the WD mass
(orbital period) can be determined, the orbital period (WD mass) can be
inferred with this relation and then the inclination angle can be constrained
with the binary mass function. In addition, we can also use this relation to
constrain the CE ejection efficiency and find that a relative large CE ejection
efficiency is favoured. If both the WD and sdB star masses can be determined,
the critical mass ratios of dynamically unstable mass transfer for RG binaries
can also be constrained.
