  We demonstrate that the entanglement in a class of two-mode non-Gaussian
states obtained by subtracting photons from Gaussian twin beams can be bounded
from above and from below by functionals of the second moments only. Knowledge
of the covariance matrix thus suffices for an entanglement quantification with
appreciable precision. The absolute error in the entanglement estimation scales
with the non-Gaussianity of the considered states.
