  We study the CMB constraints on two Dark Energy models described by scalar
fields with different Lagrangians, namely a Klein-Gordon and a Born-Infeld
field. The speed of sound of field fluctuations are different in these two
theories, and therefore the predictions for CMB and structure formation are
different. Employing the WMAP data on CMB, we make a likelihood analysis on a
grid of theoretical models. We constrain the parameters of the models and
compute the probability distribution functions for the equation of state. We
show that the effect of the different sound speeds affects the low multipoles
of CMB anisotropies, but is at most marginal for the class of models studied
here.
