White dwarfs are among the most exotic objects in the Universe. They are the typical end products of most main sequence stars, like the Sun. Therefore, they are very numerous in the sky. White dwarfs have the size of the Earth but they are as massive as the Sun.  Because of this they are very compact objects and their physical properties are quite different from those of normal stars. For example, the radius of a white dwarf decreases for increasing mass. In addition, if white dwarfs exceed a certain mass (~1.4Msun) they explode as type Ia supernovae. By analyzing these supernovae it was found that the expansion of the Universe is accelerating, a discovery which was awarded the Nobel prize in physics in 2011.

Thanks to modern large scale surveys, the number of identified white dwarfs has increased dramatically during the last two decades. However, these surveys are subject to complicated target selection algorithms. Thus, it becomes very difficult to asses how the observational biases affect the observed populations.

Recently, the LSS-GAC survey, which makes use of the LAMOST telescope, started operations in China. The main advantage of the LSS-GAC over previous surveys is that it follows a simple yet non-trivial selection criteria. Thus, not only the observed population of LSS-GAC white dwarfs is less affected by selection effects, but also it can be used to unveil the underlying population. This allowed us, for example, to derive reliable values of the space density and birth rate of white dwarfs. In addition, we provided important observational evidence for the existence of an excess of massive white dwarfs, which cannot be understood by only considering single star evolution (see figure 1). We proposed that a sizable fraction of these massive white dwarfs are the products of mergers of two lower-mass white dwarfs in binary systems. If our hypothesis is correct, then the merger of double white dwarf binaries in the Galaxy must be much higher than expected, which may have strong implications in the production of type Ia supernovae.

Figure. Simulated white dwarf mass functions (grey solid lines) resulting from four different models invoking single star evolution. The observed LSS-GAC white dwarf mass function is shown as a black solid line. The fraction of massive WDs (>0.8 Msun) for both the observed and the simulated samples are provided in the top right of each panel. It becomes clear that no model can reproduce the observed excess of massive white dwarfs. We argue this feature may arise as a consequence of white dwarf binary mergers.