  In this paper, we study an extension of the standard model with a vector-like
generation of leptons. This model provides a viable dark matter candidate and a
possibility to enhance the Higgs decay rate into a pair of photons. We evaluate
constraints from electroweak precision tests and from vacuum stability, and
find that the latter provide an upper limit on the lepton Yukawa couplings. A
large enhancement of the Higgs di-photon rate can therefore only be obtained
when the mass of the lightest charged lepton is close to the LEP limit. The
relic density constraint suggests a co-annihilation scenario with a small mass
difference between the lightest charged and neutral leptons, which also weakens
the LEP limit on the lightest charged lepton mass and allows for larger Higgs
di-photon decay rates. Cross sections for direct detection of the dark matter
candidate are calculated, and prospects for detecting the new particles at the
LHC are discussed briefly.
