The path to Z And-type outbursts: The case of V426 Sagittae (HBHA 1704-05)

Context. The star V426 Sge (HBHA 1704-05), originally classified as an emission-line object and a semi-regular variable, brightened at the beginning of August 2018, showing signatures of a symbiotic star outburst. Aims. We aim to confirm the nature of V426 Sge as a classical symbiotic star, determine the photometric ephemeris of the light minima, and suggest the path from its 1968 symbiotic nova outburst to the following 2018 Z And-type outburst. Methods. We re-constructed an historical light curve (LC) of V426 Sge from approximately the year 1900, and used original low- (R & x2004;similar to & x2004;500-1500; 330-880 nm) and high-resolution (R & x2004;similar to & x2004;11 & x2006;000-34 000; 360-760 nm) spectroscopy complemented with Swift-XRT and UVOT, optical UBVRCIC and near-infrared JHKL photometry obtained during the 2018 outburst and the following quiescence. Results. The historical LC reveals no symbiotic-like activity from similar to 1900 to 1967. In 1968, V426 Sge experienced a symbiotic nova outburst that ceased around 1990. From approximately 1972, a wave-like orbitally related variation with a period of 493.4 +/- 0.7 days developed in the LC. This was interrupted by a Z And-type outburst from the beginning of August 2018 to the middle of February 2019. At the maximum of the 2018 outburst, the burning white dwarf (WD) increased its temperature to greater than or similar to 2 x 10(5) K, generated a luminosity of similar to 7 x 10(37)& x2006;(d/3.3 & x2006;kpc)(2) erg & x2006;s(-1) and blew a wind at the rate of similar to 3 x 10(-6) M-circle dot yr(-1). Our spectral energy distribution models from the current quiescent phase reveal that the donor is a normal M4-5 III giant characterised with T-eff & x2004;similar to & x2004;3400 K, R-G & x2004;similar to & x2004;106 & x2006;(d/3.3 & x2006;kpc) R-circle dot and L-G & x2004;similar to & x2004;1350 & x2006;(d/3.3 & x2006;kpc)(2) L-circle dot and the accretor is a low-mass similar to 0.5 M-circle dot WD. Conclusions. During the transition from the symbiotic nova outburst to the quiescent phase, a pronounced sinusoidal variation along the orbit develops in the LC of most symbiotic novae. The following eventual outburst is of Z And-type, when the accretion by the WD temporarily exceeds the upper limit of the stable burning. At this point the system becomes a classical symbiotic star.