DASCH Discovery of A Possible Nova-like Outburst in A Peculiar Symbiotic Binary

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We present photometric and spectroscopic observations of a peculiar variable (designated DASCH J075731.1+201735 or J0757) discovered from our DASCH project using the digitized Harvard College Observatory archival photographic plates. It brightened by about 1.5 magnitudes in B within a year starting in 1942, and then slowly faded back to its pre-outburst brightness from 1943 to the 1950s. The mean brightness level was stable before and after the outburst, and ellipsoidal variations with a period of P=119.18±0.07 days are seen, suggesting that the star is tidally distorted. Radial-velocity measurements indicate that the orbit is nearly circular (e=0.02±0.01) with a spectroscopic period that is the same as the photometric period. The binary consists of a 1.1±0.3 M M0III star, and a 0.6±0.2 M companion, very likely a white dwarf (WD). Unlike other symbiotic binaries, there is no sign of emission lines or a stellar wind in the spectra. With an outburst timescale of 10 years and estimated B band peak luminosity MB 0.7, J0757 is different from any other known classic or symbiotic novae. The most probable explanation of the outburst is Hydrogen shell-burning on the WD, although an accretion-powered flare cannot be ruled out.