Context :

Aims : We report on an estimate of the bar pattern speed \Omega _ { p } for the low surface brightness spiral galaxy UGC 628 .

Methods : We applied the Tremaine-Weinberg method to high resolution H \alpha velocity and integrated emission maps of this dark matter dominated galaxy .
Observations were made at the CFHT using the optical Fabry-Perot interferometer , FaNTOmM .

Results : The Tremaine-Weinberg method estimates a bar pattern speed of ( 11.3 \pm 2.0 ) km s ^ { -1 } kpc ^ { -1 } for UGC 628 , which is among the lowest values found for a spiral galaxy .
The corotation radius R _ { c } of the bar and the gaseous disc is R _ { c } = 9.8 ^ { +2.9 } _ { -2.0 } kpc , implying a ratio \cal R = R _ { c } / a _ { B } of 2.0 ^ { +0.5 } _ { -0.3 } , where a _ { B } is the bar radius .
The ratio is well beyond the usual range of values , 1.0 \leq { \cal R } \leq 1.4 , found for fast bars of high surface brightness barred galaxies .
It implies that the bar in UGC 628 is slow .

Conclusions : As shown through the use of numerical simulations , fast bars survive when the inner mass distribution of galaxies is dominated by the baryons over the dark matter .
Our result suggests that the presence of slow bars in galaxies is likely related to the dominance of dark matter over the mass distribution .