A new model atmosphere analysis for the peculiar DQ white dwarf discovered by Carollo et al .
( 2002 ) is presented .
The effective temperature and carbon abundance have been estimated by fitting both the photometric data ( UB _ { J } VR _ { F } I _ { N } JHK ) and a low resolution spectrum ( 3500 < \lambda < 7500 Å ) with a new model grid for helium-rich white dwarfs with traces of carbon ( DQ stars ) .
We estimate T _ { eff } \simeq 5120 \pm 200 K and log [ C/He ] \simeq - 5.8 \pm 0.5 , which make GSC2U J131147.2+292348 the coolest DQ star ever observed .
This result indicates that the hypothetical transition from C _ { 2 } to C _ { 2 } H molecules around T _ { eff } = 6000 K , which was inferred to explain the absence of DQ stars at lower temperatures , needs to be reconsidered .