The orbital period of the V Sge star candidate QU Carinae

Close binary supersoft X-ray sources (CBSS) are considered strong candidates to Type Ia supernova progenitors, but very few CBSS are known in our Galaxy. The Galactic counterparts of the CBSS may be the V Sge stars, not detected in X-rays due to the strong absorption by the interstellar gas. Nevertheless, the number of members in the V Sge class is still small. In order to find new members, we selected QU Car for photometric and spectroscopic observations. The orbital period of this system was published in the literature as 10.9 h, determined from radial velocity data taken in 1979–1980, but posterior analysis of data taken in 2006–2007 did not confirm this period. We analysed the high variability of its emission line profiles with the temporal variance spectrum (TVS) technique. Besides, we recovered the 10.9 h orbital period from the radial velocities of the He ii 4686 Å emission line and, for the first time, detected what may be the orbital modulation in the photometric data. This photometric modulation is present only in the lower brightness state data, when the flickering is attenuated. The inclusion of QU Car in the V Sge class is supported by many features like high/low states, strong winds, nebular lines and He ii 4686 Å/Hβ line ratios. However, the non-detection of the characteristic O vi 3811–34 Å lines in its spectrum claims against this classification. These lines, though, may be highly variable so additional spectra analysed with the TVS technique can, possibly, solve this question.