Accurate pre- and post-eruption orbital periods for the dwarf/classical nova V1017 Sgr

V1017 Sgr is a classical nova (in 1919) that displayed an earlier dwarf nova eruption (in 1901), and two more dwarf nova events (in 1973 and 1991). Previous work on this bright system in quiescence (V=13.5) has only been a few isolated magnitudes, a few spectra, and an ambiguous claim for an orbital period of 5.714 days as based on nine radial velocities. To test this period, we have collected 2896 magnitudes (plus 53 in the literature) in the UBVRIJHKL bands from 1897 to 2016, making an essentially complete photometric history of this unique cataclysmic variable. We find that the light curve in all bands is dominated by the ellipsoidal modulations of a G giant companion star, with a post-eruption (after the 1919 nova event) orbital period of 5.786290 +- 0.000032 days. This is the longest period for any classical nova, the accretion must be powered by the nuclear evolution of the companion star, and the dwarf nova events occur only because the outer parts of the large disk are cool enough to be unstable. Furthermore, we measure the pre-eruption orbital period (from 1907 to 1916), and there is a small steady period change in quiescence. The orbital period has decreased by 273 +- 61 parts-per-million across the 1919 eruption, with the significance of the period change being at the 5.7-sigma confidence level. This is startling and mystifying for nova-theory, because the three known period change effects cannot account for a period decrease in V1017 Sgr, much less one of such a large size.