QU Car is listed in cataclysmic variable star catalogues as a nova-like variable .
This little-studied , yet bright interacting binary is re-appraised here in the light of new high-quality ultraviolet ( UV ) interstellar line data obtained with STIS on board the Hubble Space Telescope .
The detection of a component of interstellar absorption at a mean LSR velocity of - 14 km s ^ { -1 } indicates that the distance to QU Car may be \sim 2 kpc or more – a considerable increase on the previous lower-limiting distance of 500 pc .
If so , the bolometric luminosity of QU Car could exceed 10 ^ { 37 } ergs s ^ { -1 } .
This would place this binary in the luminosity domain occupied by known compact-binary supersoft X-ray sources .
Even at a 500 pc , QU Car appears to be the most luminous nova-like variable known .

New intermediate dispersion optical spectroscopy of QU Car spanning 3800–7000 Å is presented .
These data yield the discovery that C iv \lambda \lambda 5801,12 is present as an unusually prominent emission line in an otherwise low-contrast line spectrum .
Using measurements of this and other lines in a recombination line analysis , it is shown that the C/He abundance as proxied by the n ( C ^ { 4 + } ) /n ( He ^ { 2 + } ) ratio may be as high as 0.06 ( an order of magnitude higher than the solar ratio ) .
Furthermore , the C/O abundance ratio is estimated to be greater than 1 .
These findings suggest that the companion in QU Car is a carbon star .
If so , it would be the first example of a carbon star in such a binary .
An early-type R star best matches the required abundance pattern and could escape detection at optical wavelengths provided the distance to QU Car is \sim 2 kpc or more .