This proposal is a resubmission of an accepted RXTE AO 11 proposal. Our aim is to test evolutionary and type I burst models with the observed type I X-ray burst behaviour of the bright ultra-compact binary system 4U 1820-30. The short orbital period suggests that the companion white dwarf is hydrogen deficient, with evolutionary models predicting a composition of either pure helium or a small amount of hydrogen (~5-35% by mass). We propose to compare observed type I burst properties with detailed X-ray burst models, constraining the composition of the donor star and testing evolutionary models.