  The white dwarf luminosity function, which provides information about their
cooling, has been measured with high precision in the past few years.
Simulations that include well known Standard Model physics give a good fit to
the data. This leaves little room for new physics and makes these astrophysical
objects a good laboratory for testing models beyond the Standard Model. It has
already been suggested that white dwarfs might provide some evidence for the
existence of axions. In this work we study the constraints that the white dwarf
luminosity function puts on physics beyond the Standard Model involving new
light particles (fermions or bosons) that can be pair-produced in a white dwarf
and then escape to contribute to its cooling. We show, in particular, that we
can severely constrain the parameter space of models with dark forces and light
hidden sectors (lighter than a few tens of keV). The bounds we find are often
more competitive than those from current lab searches and those expected from
most future searches.
