In this paper we present and discuss our time-resolved photometry of an eclipsing recurrent nova , U Sco , during an outburst in 1999 , which was conducted from immediately after the optical maximum to the final fading toward the quiescence .
In the first-ever complete light-curve , a few primary and secondary eclipses of the binary system were detected , and the timings of the minima were determined .
We found that the eclipses showed no totality during the outburst .
The depth of the primary eclipses was 0.4–0.8 mag , much shallower than that in quiescence .
In the plateau phase , very little irradiation ( \leq 0.1 mag ) was observed in the orbital light curve , which implies the existence and a large flaring rim of the accretion disk during the outburst .
The minima of the eclipses were detected at earlier orbital phases for the predicted ephemerides .
Thus , we obtained an orbital period change of the binary system as \dot { P } / P = -1.7 ( \pm 0.7 ) \times 10 ^ { -6 } ~ { } { yr ^ { -1 } } from the O - C .
Assuming that this period change is a result of the conservative mass transfer between the component stars , its mass-transfer rate reaches \dot { M } = 2.4 ( \pm 1.0 ) \times 10 ^ { -6 } ~ { } M _ { \odot } ~ { } { yr ^ { -1 } } for a 1.37 M _ { \odot } white dwarf and a 2.0 M _ { \odot } mass-donor companion , which is too high to cause shell flashes , even on a massive white dwarf .
Therefore , this large rate of the period change strongly indicates a non-conservative mass transfer in the binary system .