  A study of the temperature and field dependence of the penetration depth
\lambda of the superconductor RbFe_2As_2 (T_c=2.52 K) was carried out by means
of muon-spin rotation measurements. In addition to the zero temperature value
of the penetration depth \lambda(0)=267(5) nm, a determination of the upper
critical field B_c2(0)=2.6(2) T was obtained. The temperature dependence of the
superconducting carrier concentration is discussed within the framework of a
multi-gap scenario. Compared to the other "122" systems which exhibit much
higher Fermi level, a strong reduction of the large gap BCS ratio 2\Delta/k_B
T_c is observed. This is interpreted as a consequence of the absence of
interband processes. Indications of possible pair-breaking effect are also
discussed.
