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V1017 Sgr is a classical nova (in 1919) that displayed an earlier dwarf novaeruption (in 1901), and two more dwarf nova events (in 1973 and 1991). Previouswork on this bright system in quiescence (V=13.5) has only been a few isolatedmagnitudes, a few spectra, and an ambiguous claim for an orbital period of5.714 days as based on nine radial velocities. To test this period, we havecollected 2896 magnitudes (plus 53 in the literature) in the UBVRIJHKL bandsfrom 1897 to 2016, making an essentially complete photometric history of thisunique cataclysmic variable. We find that the light curve in all bands isdominated by the ellipsoidal modulations of a G giant companion star, with apost-eruption (after the 1919 nova event) orbital period of 5.786290 +-0.000032 days. This is the longest period for any classical nova, the accretionmust be powered by the nuclear evolution of the companion star, and the dwarfnova events occur only because the outer parts of the large disk are coolenough 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 1919eruption, with the significance of the period change being at the 5.7-sigmaconfidence level. This is startling and mystifying for nova-theory, because thethree known period change effects cannot account for a period decrease in V1017Sgr, much less one of such a large size.

Accurate Pre-Eruption and Post-Eruption Orbital Periods for the Dwarf/Classical Nova V1017 Sgr