  We demonstrate the emission of highly indistinguishable photons from a
quasi-resonantly pumped coupled quantum dot--microcavity system operating in
the regime of cavity quantum electrodynamics. Changing the sample temperature
allows us to vary the quantum dot--cavity detuning, and on spectral resonance
we observe a three-fold improvement in the Hong--Ou--Mandel interference
visibility, reaching values in excess of 80\%. Our measurements off-resonance
allow us to investigate varying Purcell enhancements, and to probe the
dephasing environment at different temperatures and energy scales. By
comparison with our microscopic model, we are able to identify pure-dephasing
and not time-jitter as the dominating source of imperfections in our system.
