  Massive scalar test fields around Kerr black holes can form quasi-bound
states with complex frequencies. Some of these states decay in time, but some
others -- in the superradiant regime -- grow, causing a superradiant
instability. Precisely at the threshold between decaying and growing modes,
there exist bound states with real frequency, known as scalar clouds. Fully
non-linear counterparts of these clouds have been shown to yield Kerr black
holes with scalar hair. Here, we consider massive, (electrically) charged, test
scalar and Proca fields on the background of Reissner-Nordstr\"om black holes.
By computing the quasi-bound state frequencies, we establish that no such
states exist in the superradiant regime for the Proca field -- a similar
behavior to that known for scalar fields. But decaying quasi-bound states with
an arbitrarily small imaginary part of the frequency exist and thus which are
arbitrarily long lived. In the limit of vanishing imaginary part of the
frequency, the field does not trivialize and we dub the corresponding
configurations as marginal (charged) scalar or Proca clouds, since they are
only marginally bound. We comment on the possible existence of non-linear
counterparts to these marginal clouds.
