| Abstract
| - We present high-time-resolution photometry of the dwarf nova IY UMa using the S-Cam2 superconducting tunnel junction device attached to the 4.2-m William Herschel Telescope on La Palma. Exploiting the well-defined white dwarf and hotspot eclipse features, we derive an updated orbital ephemeris for IY UMa and an orbital period of 0.073 908 97(5) d. A white dwarf ingress/egress duration of 31 ± 2 s along with the contact phases of the bright-spot gives M1= 0.79 ± 0.04 M⊙ and M2= 0.10 ± 0.01 M⊙, corresponding to a mass ratio of q= 0.125 ± 0.008. The white dwarf eclipse width of ΔφWD= 0.0637 then implies . A curious rise with a duration of 30 ± 2 s is present in the orbital light curves during all three observed eclipses. It occurs between the end of white dwarf ingress and hotspot ingress, and is blue in colour. We suggest that the source of this light lies in the buried part of the gas stream, resulting in a compact, hot impact cavity.
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