  XTE J1701-407 is a new transient X-ray source discovered on June 8th, 2008.
More than one month later it showed a rare type of thermonuclear explosion: a
long type I X-ray burst. We report herein the results of our study of the
spectral and flux evolution during this burst, as well as the analysis of the
outburst in which it took place. We find an upper limit on the distance to the
source of 6.1 kpc by considering the maximum luminosity reached by the burst.
We measure a total fluence of 3.5*10^{-6} erg/cm^2 throughout the ~20 minutes
burst duration and a fluence of 2.6*10^{-3} erg/cm^2 during the first two
months of the outburst. We show that the flux decay is best fitted by a power
law (index ~1.6) along the tail of the burst. Finally, we discuss the
implications of the long burst properties, and the presence of a second and
shorter burst detected by Swift ten days later, for the composition of the
accreted material and the heating of the burning layer.
