  Anomalously light fermionic partners of the top quark often appear in
explicit constructions, such as the 5d holographic models, where the Higgs is a
light composite pseudo Nambu-Goldstone boson and its potential is generated
radiatively by top quark loops. We show that this is due to a structural
correlation among the mass of the partners and the one of the Higgs boson.
Because of this correlation, the presence of light partners could be essential
to obtain a realistic Higgs mass.
  We quantitatively confirm this generic prediction, which applies to a broad
class of composite Higgs models, by studying the simplest calculable framework
with a composite Higgs, the Discrete Composite Higgs Model. In this setup we
show analytically that the requirement of a light enough Higgs strongly
constraints the fermionic spectrum and makes the light partners appear.
  The light top partners thus provide the most promising manifestation of the
composite Higgs scenario at the LHC. Conversely, the lack of observation of
these states can put strong restrictions on the parameter space of the model. A
simple analysis of the 7-TeV LHC searches presently available already gives
some non-trivial constraint. The strongest bound comes from the exclusion of
the 5/3-charged partner. Even if no dedicated LHC search exists for this
particle, a bound of 611 GeV is derived by adapting the CMS search of
bottom-like states in same-sign dileptons.
