  Many models have been studied that contain more than one species of dark
matter and some of these couple the Cold Dark Matter (CDM) to a light scalar
field. In doing this we introduce additional long range forces, which in turn
can significantly affect our estimates of cosmological parameters if not
properly accounted for. It is, therefore, important to study these models and
their resulting cosmological implications. We present a model in which a
fraction of the total cold dark matter density is coupled to a scalar field. We
study the background and perturbation evolution and calculate the resulting
Cosmic Microwave Background anisotropy spectra. The greater the fraction of
dark matter coupled to the scalar field and the stronger the coupling strength,
the greater the deviation of the background evolution from LCDM. Previous work,
with a single coupled dark matter species, has found an upper limit on the
coupling strength of order O(0.1). We find that with a coupling of this
magnitude more than half the dark matter can be coupled to a scalar field
without producing any significant deviations from LCDM.
