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The ultracompact low-mass X-ray binary 4U 1820-30 situated in the globularcluster NGC 6624 has an orbital period of only $\approx$11.4 min which likelyimplies a white dwarf companion. The observed X-ray bursts demonstrate aphotospheric radius expansion phase and therefore are believed to reach theEddington luminosity allowing us to estimate the mass and the radius of theneutron star (NS) in this binary. Here we re-analyse all Rossi X-ray TimingExplorer observations of the system and confirm that almost all the bursts tookplace during the hard persistent state of the system. This allows us to use therecently developed direct cooling tail method to estimate the NS mass andradius. However, because of the very short, about a second, duration of thecooling tail phases that can be described by the theoretical atmosphere models,the obtained constraints on the NS radius are not very strict. Assuming a purehelium NS atmosphere we found that the NS radius is in the range 10-12 km, ifthe NS mass is below 1.7 $M_\odot$, and in a wider range of 8-12 km for ahigher 1.7-2.0 $M_\odot$ NS mass. The method also constrains the distance tothe system to be 6.5$\pm$0.5 kpc, which is consistent with the distance to thecluster. For the solar composition atmosphere, the NS parameters are in strongcontradiction with the generally accepted range of possible NS masses andradii.

Basic parameters of the helium accreting X-ray bursting neutron star in 4U 1820-30