  We have analysed deep R-band images, down to a limiting surface brightness of
26.5 R-mag arcsec$^{-2}$ (equivalent to ~28 B-mag arcsec$^{-2}$), of 5 cD
galaxies to determine the shape of the surface brightness profiles of their
extended stellar envelopes. Both de Vaucouleurs R^{1/4} model and Sersic's
R^{1/n} model, on their own, provide a poor description of the surface
brightness profiles of cD galaxies. This is due to the presence of outer
stellar envelopes, thought to have accumulated over the merger history of the
central cluster galaxy and also from the tidal stripping of galaxies at larger
cluster radii. We therefore simultaneously fit two Sersic functions to measure
the shape of the inner and outer components of the cD galaxies. We show that,
for 3 out of our 5 galaxies, the surface brightness profiles are best fit by an
inner Sersic model, with indices n~1-6, and an outer exponential component. For
these systems, the galaxy-to-envelope size ratio is 0.1 - 0.4 and the
contribution of the stellar envelope to the total R-band light (i.e. galaxy +
envelope) is around 60 to 80 per cent (based on extrapolation to a 300 kpc
radius). The exceptions are NGC 6173, for which our surface brightness profile
modelling is consistent with just a single component (i.e. no envelope) and NGC
4874 which appears to have an envelope with a de Vaucouleurs, rather than
exponential, profile.
