  If primordial black holes constitute the dark matter, stars forming in
dark-matter dominated environments with low velocity dispersions, such as
ultra-faint dwarf galaxies, may capture a black hole at birth. The capture
probability is non-negligible for primordial black holes of masses around
$10^{20}$g, and increases with stellar mass. Moreover, infected stars are
turned into virtually invisible black holes on cosmologically short timescales.
Hence, the number of observed massive main-sequence stars in ultra-faint dwarfs
should be suppressed if the dark matter was made of asteroid-mass primordial
black holes. This would impact the measured mass distribution of stars, making
it top-light (i.e. depleted in the high-mass range). Using simulated data that
mimic the present-day observational power of telescopes, we show that already
existing measurements of the mass function of stars in local ultra-faint dwarfs
could be used to constrain the fraction of dark matter composed of primordial
black holes in the -- currently unconstrained -- mass range of
$10^{19}-10^{21}$g.
