  Some gaps in the mosaic of binary star evolution have recently been filled by
the discoveries of helium-core white dwarf progenitors (often called extremely
low mass (ELM) white dwarfs) as stripped cores of first-giant branch objects.
Two varieties can be distinguished. One class is made up by SB1 binaries,
companions being white dwarfs as well, another class, the so-called EL CVn
stars, are composite spectrum binaries, with A-Type companions. Pulsating stars
are found among both classes. A riddle is posed by the apparently single
objects. There is a one-to-one correspondence of the phenomena found for these
new classes of star to those observed for sdB stars. In fact, standard
evolutionary scenarios explain the origin of sdB stars as red giants that have
been stripped close to the tip of first red giant branch. A subgroup of
subluminous B stars can also be identified as stripped helium-cores of red
giants. They form an extension of the ELM sequence to higher temperatures.
Hence low mass white dwarfs of helium cores and sdB stars in binaries are close
relatives in terms of stellar evolution.
