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We obtained an optical spectrum of a star we identify as the opticalcounterpart of the M31 Chandra source CXO J004318.8+412016, because ofprominent emission lines of the Balmer series, of neutral helium, and a He IIline at 4686 Angstrom. The continuum energy distribution and the spectralcharacteristics demonstrate the presence of a red giant of K or earlierspectral type, so we concluded that the binary is likely to be a symbioticsystem. CXO J004318.8+412016 has been observed in X-rays as a luminoussupersoft source (SSS) since 1979, with effective temperature exceeding 40 eVand variable X-ray luminosity, oscillating between a few times 10(35) erg/s anda few times 10(37) erg/s. The optical, infrared and ultraviolet colors of theoptical object are consistent with an an accretion disk around a compact objectcompanion, which may either be a white dwarf, or a black hole, depending on thesystem parameters. If the origin of the luminous supersoft X-rays is theatmosphere of a white dwarf that is burning hydrogen in shell, it is as hot andluminous as post-thermonuclear flash novae, yet no major optical outburst hasever been observed, suggesting that the white dwarf is very massive (m>1.2M(sol)) and it is accreting and burning at the high rate(mdot>10(-8)M(sol)/year) expected for type Ia supernovae progenitors. In thiscase, the X-ray variability may be due to a very short recurrence time of onlymildly degenerate thermonuclear flashes.

CXO J004318.8+412016, a steady supersoft X-ray source in M 31