  Context. Sulfur (S) is one of the lesser-studied $\alpha$-elements. Published
investigations of its behavior have so far focused on local stars, and only a
few clusters of the Milky Way have been considered to study this topic. We aim
to study the S content of the globular cluster Ruprecht 106 -- which has never
before been studied for this purpose, but is known to present low levels of the
[$\alpha$/Fe] abundance ratio -- and the open cluster Trumpler 5. The only star
studied so far in Trumpler 5 shows an unexpectedly low abundance of S. Aims.
With this work, we aim to provide the first S abundance in Ruprecht 106 and to
investigate the S content of Trumpler 5 with a larger sample of stars. The open
cluster Trumpler 20 is considered as a reference object. Methods. We performed
a standard abundance analysis based on 1D model atmospheres in local
thermodynamical equilibrium (LTE) and on high-resolution and
high-signal-to-noise-ratio UVES-slit and UVES/FLAMES spectra. We also applied
corrections for nonLTE. The metallicities of the targets were obtained by
studying equivalent widths. Sulfur abundances were derived from multiplets 1,
6, and 8 by spectrosynthesis. Results. We find that the metallicities of
Ruprecht 106 and Trumpler 5 are [Fe/H]= -1.37+/-0.11 and [Fe/H]= -0.49+/-0.14,
respectively. Ruprecht 106 is less S-rich than the other Galactic clusters at
similar metallicity. The low S content of Ruprecht 106, [S/Fe]NLTE=
-0.52+/-0.13, is consistent with its shortage of $\alpha$-elements. This
supports an extra-galactic origin of this cluster. We obtained a new and more
robust S content value of Trumpler 5 of about [S/Fe]NLTE= 0.05+/-0.20.
According to our results, Trumpler 5 follows the trend of the Galactic disk in
the [S/Fe]LTE versus [Fe/H] diagram. Our results for Trumpler 20, of namely
[Fe/H]= 0.06+/-0.15 and [S/Fe]NLTE= -0.28+/-0.21, are in agreement with those
in the literature.
