r zero-age main-sequence stars ([@web85]). This may pose a requirement $M_{\rm MS} \gtrsim 1.4 M_\odot$. We estimate the most likely companion mass of 1.4—1.6 $M_\odot$ from equation (11) in Hachisu et al. (1999b).

If the distance to U Sco is $\sim$ 6.0—8.0 kpc, it is located $\sim$ 2.3—3.0 kpc above the Galactic plane ($b=22\arcdeg$). The zero-age masses of the progenitor system to U Sco are rather massive (e.g., $8.0 ~M_\odot + 2.5 ~M_\odot$ from Hachisu et al. 1999b) and it is unlikely that such massive stars were born in the halo. Some normal B-type main-sequence stars have been found in the halo (e.g., PG0009+036 is located $\sim$ 5 kpc below the Galactic disk, [@smt96]), which were ejected from the Galactic disk because of their relatively high moving velocities $\sim$100—200 km s$^{-1}$. The radial velocity of U Sco is not known but it is suggested that the $\gamma$-velocity is $\sim$50—100 km s$^{-1}$ from the absorption line velocities ([@joh92]; [@sch95]). If so, it seems likely that U Sco was ejected from the Galactic disk with a vertical velocity faster than $\sim 20$ km s$^{-1}$ and has reached at the present place within the main-sequence lifetimes of a $\sim 3.0 M_\odot$ star ($\sim 3.5\times 10^8$ yr).

Now, we can understand the current evolutionary status and a further evolution of U Sco system. The white dwarf has a mass $1.37 \pm 0.01 M_\odot$. It is very likely that the WD has reached such a large mass by mass accretion. In fact the WD is currently increasing the mass of the helium layer at a rate of $\dot M_{\rm He} \sim 1.0 \times 10^{-7} M_\odot$ yr$^{-1}$ ([@hac2000]). We then predict that the WD will evolve as follows. When the mass of the helium layer reaches a critical mass after many cycles of recurrent nova outbursts, a helium shell flash will occur. Its strength is as weak as those of AGB stars because of the high mass accretion rate ([@nom82]). A part of the helium layer will be blown off in the wind, but virtually all of the helium layer will be burnt into carbon-oxygen