  Recently, a large number of low-mass (<0.30 M_sun) helium white dwarfs (He
WDs) have been discovered as a result of several surveys campaigns such as
WASP, ELM, Kepler or SDSS. The far majority of them have as companion another
compact object. There appears to be discrepancies between current theoretical
modelling of low-mass He WDs and a number of key observational cases indicating
that some details of their formation scenario yet remain to be understood.
