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We present X-ray through submillimeter observations of the classical RV Tauri(RVb-type) variable U Mon, a post-AGB binary with a circumbinary disk (CBD).Our SMA observations indicate a CBD diameter of $\lesssim$550 au. OurXMM-Newton observations make U Mon the first RV Tauri variable detected inX-rays. The X-ray emission is characteristic of a hot plasma ($\sim$10 MK),with L$_{X}=5\times10^{30}{\rm erg}~{\rm s}^{-1}$, and we consider its possibleorigin from U Mon, its companion, and/or binary system interactions. CombiningDASCH and AAVSO data, we extend the time-series photometric baseline back tothe late 1880s and find evidence that U Mon has secular changes that appear torecur on a timescale of $\sim$60 yr, possibly caused by a feature in the CBD.From literature radial velocities we find that the binary companion is a$\sim$2 M$_{\odot}$ A-type main-sequence star. The orientation of the binary'sorbit lies along our line of sight ($\omega = 95^\circ$), such that apastroncorresponds to photometric RVb minima, consistent with the post-AGB starbecoming obscured by the near side of the CBD. In addition, we find the size ofthe inner-CBD hole ($\sim$4.5-9 au) to be comparable to the binary separation,implying that one or both stars may interact with the CBD at apastron. Theobscuration of the post-AGB star implicates the companion as the likely sourceof the enhanced H$\alpha$ observed at RVb minima and of the X-ray emission thatmay arise from accreted material.

astrophysical journal/the astrophysical journal

Multiwavelength Observations of the RV Tauri Variable System U  Monocerotis: Long-Term Variability Phenomena That Can Be Explained by Binary  Interactions with a Circumbinary Disk