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

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 mag in B within a year starting in 1942, and then slowly faded back to its pre-outburst brightness from 1943 to 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 M_B ~ 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.