  The pulsating hydrogen atmosphere white dwarf star G 117-B15A has been
observed since 1974. Its main pulsation period at 215.19738823(63) s, observed
in optical light curves, varies by only (5.12+/-0.82)x10^{-15} s/s and shows no
glitches, as pulsars do. The observed rate of period change corresponds to a
change of the pulsation period by 1 s in 6.2 million years. We demonstrate that
this exceptional optical clock can continue to put stringent limits on
fundamental physics, such as constraints on interaction from hypothetical dark
matter particles, as well as to search for the presence of external substellar
companions.
