Resolve the Evans–Hellier ambiguity through emission-wing asymmetry

Characterize the phase-dependent violet-to-red emission-wing asymmetry of V844 Herculis with sufficient precision to determine whether both magnetic accretion curtains are equally visible and thereby resolve the factor-of-two ambiguity in the white-dwarf spin period.

Background

The Evans–Hellier ambiguity arises because a double-peaked optical pulse can occur when both magnetic accretion curtains are visible, causing the observed photometric period to be half the white-dwarf spin period. A variation in the violet-to-red flux ratio of emission-line wings over a spin cycle can diagnose whether the accretion flow is viewed asymmetrically.

For V844 Herculis, the mean violet-to-red ratio is close to unity, but the measurements are noisy. The authors state that the current spectroscopy cannot determine the degree of symmetry accurately enough to establish whether the observed 29.3-minute modulation is the spin period or its first harmonic.

References

With the current data, we cannot constrain the degree of symmetry of the line wings in V844~Her with sufficient accuracy to resolve the Evans and Hellier ambiguity.

— V844 Herculis: An Exceptional Intermediate Polar Hiding in an Ordinary Dwarf Nova  (2609.26530 - Greiveldinger et al., 22 Sep 2026) in Section 5.2, “Spin Period”