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We report an optical study of the excitonic magnetic polaron in Cdl.,Mn,Te/ Cdl-yMnyTe quantum well. The polaron energy is measured as the Stokes shift observed when the luminescence is selectively excited in the low energy part of the elhl exciton density of states. For quantum wells with a Mn concentration as small as x=0.07 the polaron energy decreases rapidly as the quantum well width increases. This dependence which is consistent with the non-observation of the magnetic polaron in bulk material is well accounted for by a simple theory which assumes a weak initial localisation of the exciton. The mobility edge separating localised and delocalised exciton states is found experimentally to be very close to the maximum of the density of states. For quantum wells with higher Mn concentrations (~4. 1 4) the polaron energy decrease with increasing well width is much more gradual and the polaron is also observed for excitation on the high energy side of the density of states. This implies a stronger localisation of the heavy hole by alloy fluctuations in this case.