My talk consists of two parts. First, I will give a very brief outlook of why and how people study neutron stars in general and their thermal evolution in particular. In the second part I will provide a more specific example and discuss in details a particular case of low-mass X-ray binary HETE J1900.1–2455. After ~10 years of accretion neutron star in this transient system is still rather cold, which means that it either have high heat capacity or undergoes intensive neutrino cooling. And while we were unable to disentangle these two possibilities, both of them suggest that a significant fraction of the core is not superfluid/superconductor. I will present both simple analytical estimates of the star’s heat capacity as well as a detailed investigation by means of Markov Chain Monte-Carlo technique.