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We propose to continue our very successful program to use the observed cooling of the crusts in transiently accreting neutron star X-ray binaries to probe the behavior of ultra-dense matter. Those crusts are heated due to the accretion of matter during the outbursts of those transients. After the outbursts are over the crusts cool until they are in equilibrium with the cores again. Following this cooling process for several systems has given us important insights in the structure of neutron stars comma but many uncertainties remain. Therefore it is needed to enlarge our sample to obtain better insights in the behavior of how neutron stars react to the accretion of matter. Our program combines the strengths of Chandra and XMM-Newton and has proven to be excellently suited to obtain those goals.