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 \Lambda CDM .
Previous work , with a single coupled dark matter species , has found an upper limit on the coupling strength of order \mathcal { 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 \Lambda CDM .