YY Hya and its interstellar environment
  During a search for previously unknown Galactic emission nebulae, we
discovered a faint 36' diameter Halpha emission nebula centered around the
periodic variable YY Hya. Although this star has been classified as RR-Lyr
variable, such a classification is inconsistent Gaia distance of ~450 pc. GALEX
image data also shows YY Hya to have a strong UV excess, suggesting the
existence of a hot, compact binary companion. In addition to our discovery
image data, we obtained image of the region with CHILESCOPE time-series
spectroscopy at MDM observatory. Also, we used data from various space missions
to derive an exact orbital period and a SED. We find that YY Hya is a compact
binary system containing a K dwarf star which is strongly irradiated by a hot
WD companion. The spectral characteristics of the emission lines, visible only
during maximum light of the perfectly sinusoidal optical}light curve shows
signatures much like members of the BE UMa variable family. These are post
common envelope pre-cataclysmic variables. However the companion star here is
more massive than found in other group members and the progenitor of the white
dwarf must have been a 3 to 4 Mo star. The nebula seems to be an ejected common
envelope shell with a mass in the order of one Mo and an age of 500000 years.
This makes it to be the biggest hitherto known such shell. Alignment of
neighboring nebulosities some 45' to the northeast and southwest of YY Hya
suggests that the system had strong bipolar outflows. We briefly speculate it
might be related to the 1065 BP "guest-star" reported in ancient Chinese
records as well.
