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Common envelope evolution is a fundamental ingredient in our understanding ofthe formation of close binary stars containing compact objects which includesthe progenitors of type Ia supernovae, short gamma ray bursts and most stellargravitational wave sources. To predict the outcome of common envelope evolutionwe still rely to a large degree on a simplified energy conservation equation.Unfortunately, this equation contains a theoretically rather poorly constrainedefficiency parameter ($\alpha_{\mathrm{CE}}$) and, even worse, it is unclear ifenergy sources in addition to orbital energy (such as recombination energy)contribute to the envelope ejection process. In previous works we reconstructedthe evolution of observed populations of post common envelope binaries (PCEBs)consisting of white dwarfs with main sequence star companions and foundindications that the efficiency is rather small($\alpha_{\mathrm{CE}}\simeq0.2-0.3$) and that extra energy sources are onlyrequired in very few cases. Here we used the same reconstruction tool toinvestigate the evolutionary history of a sample of observed PCEBs with browndwarf companions. In contrast to previous works, we found that the evolution ofobservationally well characterized PCEBs with brown dwarf companions can beunderstood assuming a low common envelope efficiency($\alpha_{\mathrm{CE}}=0.24-0.41$), similar to that required to understandPCEBs with main sequence star companions, and that contributions fromrecombination energy are not required. We conclude that the vast majority ofPCEBs form from common envelope evolution that can be parameterized with asmall efficiency and without taking into account additional energy sources.

Close detached white dwarf + brown dwarf binaries: further evidence for low values of the common envelope efficiency