According to a new study looking Neanderthal and Denisovan genomes with powerful statistical analysis tools:
We identify 3% of the Neanderthal genome that is putatively introgressed from ancient humans, and estimate that the gene flow occurred between 200-300 kya. . . . We also identify 1% of the Denisovan genome which was likely introgressed from an unsequenced hominin ancestor, and note that 15% of these regions have been passed on to modern humans through subsequent gene flow.
It seems to be a rule that any new species (or mostly population replacing migration wave within the same species for that matter) will receive some introgression from its predecessor species, which in turn is passed on in greatly diluted amounts to its successor species.