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The nearby galaxy NGC 300 is hosting two luminous transient supersoft X-raysources with bolometric luminosities above 3x10^38 erg/s, assuming simpleblack-body spectra with temperatures around 60-70 eV. For one of these, SSS1, aperiodic modulation of 5.4h was observed in an XMM-Newton observation from 1stof January 2001 lasting 47 ks, but not visible 6 days earlier when theluminosity was higher. We report here the detection of a new outburst from thissource, which occurred during two more recent XMM-Newton observations performedon 17 to 20 December 2016 lasting for 310 ks. The luminosity was similar as inDecember 2000, and the 0.2-2.0 keV light curve revealed again a periodicmodulation, with a period of 4.68+-0.26h, significant only in the first of thetwo observations. Taking into account the large uncertainties (the 2001 periodwas re-estimated at 5.7+-1.1h), the two values could be marginally compatible,and maybe associated with an orbital period, although the signal strength ishighly variable. Thanks to the new long exposures, an additional absorptionfeature is now visible in the spectra, that we modelled with an absorptionedge. This component decreases the bolometric luminosity below 3x10^38 erg/sand would therefore allow the presence of a white dwarf with a mass close tothe Chandrasekhar limit. The system was found in outburst in 1992, 2000, 2008,and 2016 suggesting a possible recurrence period of about 8 years. We discussviable models involving white dwarfs, neutron stars or black holes.

New outburst from the luminous supersoft source SSS1 in NGC 300 with periodic modulation