  We discuss a new method which is potentially capable of constraining
cosmological parameters using observations of giant luminous arcs in rich X-ray
clusters of galaxies. The mass profile and the mass normalization of the lenses
are determined from X-ray measurements. The method also allows to probe the
amount and equation of state of the dark energy in the universe. The analysis
of a preliminary sample of 6 luminous, relatively relaxed clusters of galaxies
strongly favours an accelerating expansion of the universe. Under the
assumption that the dark energy is in the form of a cosmological constant, the
data provide an estimate of $ \Omega_\Lambda = 1.1$ with a statistical error of
$\pm 0.2$. Including the constraint of a flat universe and an equation of state
for the dark energy $w_X \geq -1$, we obtain $\Omega_M = 0.10 \pm 0.10$ and
$w_X = -0.84 \pm 0.14$. Relaxing the prior on $w_X$, we find that the null
energy condition is satisfied at the 3-$\sigma$ confidence level.
