An evolutionary scenario for the U Scorpii
  We perform evolutionary calculations of binary stars to find progenitors of systems with parameters similar to the recurrent novae U Sco. We show that a U Sco type-system may be formed starting with an initial binary system which has a low-mass carbon-oxygen white dwarf as an accretor. Since the evolutionary stage of the secondary is not well known, we calculate sequences with hydrogen rich and helium rich secondaries. The evolution of the binary may be devided into several observable stages as: classical nova, supersoft X-ray source with hydrogen stable burning and strong wind phases, ending up with the formation of a massive white dwarf near the Chandrasekhar mass limit. We follow the chemical evolution of the secondary as well as of the matter lost from the system, and we show that observed ^{12}C/^{13}C and N/C ratios may give some information about the nature of the binary. 
Ene Ergma (Physics Department, Tartu University), Jelena Gerskevits   (Physics Department, Tartu University) and Marek J. Sa...