YY Hya and its interstellar environment
During a search for previously unknown Galactic emission nebulae, we discovered a faint 36 minute 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 about 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 solar mass star. The nebula seems to be an ejected common envelope shell with a mass in the order of one solar mass and an age of 500000 years. This makes it to be the biggest hitherto known such shell. Alignment of neighboring nebulosities some 45 minutes 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.