2025ApJ...983..145H

Millinovae are a new class of transient supersoft X-ray sources with no clear signature of mass ejection. They show similar triangle shapes of V/I band light curves with 1000 times fainter peaks than typical classical novae. Maccarone et al. regarded the prototype millinova, ASASSN-16oh, as a dwarf nova and interpreted the supersoft X-rays to originate from an accretion belt on a white dwarf (WD). Kato et al. proposed a nova model induced by high-rate mass accretion during a dwarf nova outburst; the X-rays originate from the photosphere of a hydrogen-burning hot WD, whereas the V/I band photons are from the irradiated accretion disk. Because each peak brightness differs largely from millinova to millinova, we suspect that not all millinova candidates host a hydrogen-burning WD. Based on the light-curve analysis of the classical nova KT Eri that has a bright disk, we find that the disk is more than 2 magnitudes brighter when the disk is irradiated by the hydrogen-burning WD than when not irradiated. We present the demarcation criterion for hydrogen burning to be ${I}_{{\rm{q}}}-{I}_{{\rm{\max }}}\gt 2.2$, where I_q and ${I}_{{\rm{\max }}}$ are the I magnitudes in quiescence and at maximum light, respectively. Among many candidates, this requirement is satisfied with the two millinovae in which soft X-rays were detected.