  White Dwarfs (WDs) are the final evolutionary product of the vast majority of
stars in the Universe. They are electron-degenerate structures characterized by
no stable thermonuclear activity, and their evolution is generally described as
a pure cooling process. Their cooling rate is adopted as cosmic chronometer to
constrain the age of several Galactic populations, including the disk, globular
and open clusters. By analysing high-resolution photometric data of two twin
Galactic globular clusters (M3 and M13), we find a clear-cut and unexpected
over-abundance of bright WDs in M13. Theoretical models suggest that,
consistently with the horizontal branch morphology, this over-abundance is due
to a slowing down of the cooling process in ~70% of the WDs in M13, caused by
stable thermonuclear burning in their residual hydrogen-rich envelope. This is
the first observational evidence of quiescent thermonuclear activity occurring
in cooling WDs and it brings new attention on the use of the WD cooling rate as
cosmic chronometer for low metallicity environments.
