Embedded cooling comparison between metal-free (M_c = M_z, solid lines) and polluted (M_c = 0.5 M_z, dashed lines) grain-free envelopes. The lower and upper lines show initial hydrogen-helium mass fractions of 5 and 10%, respectively.It is clear that while polluted envelopes initially accrete significant amounts of nebular gas, their contraction slows down considerably due to the dilution of the envelope.