We present new constraints on the system parameters of the SW Sextantis star DW Ursae Majoris , based on ultraviolet ( UV ) eclipse observations with the Hubble Space Telescope .
Our data were obtained during a low state of the system , in which the UV light was dominated by the hot white dwarf ( WD ) primary .
The duration of the WD eclipse allows us to set a firm lower limit on the mass ratio , q = M _ { 2 } / M _ { 1 } > 0.24 ; if q < 1.5 ( as expected on theoretical grounds ) the inclination must satisfy i > 71 ^ { \circ } .
We have also been able to determine the duration of WD ingress and egress from our data .
This allows us to constrain the masses and radii of the system components and the distance between them to be 0.67 \leq M _ { 1 } / M _ { \odot } \leq 1.06 , 0.008 \leq R _ { 1 } / R _ { \odot } \leq 0.014 , M _ { 2 } / M _ { \odot } > 0.16 , R _ { 2 } / R _ { \odot } > 0.28 and a / R _ { \odot } > 1.05 .
If the secondary follows Smith & Dhillon ’ s mass-period relation for CV secondaries , our estimates for the system parameters become M _ { 1 } / M _ { \odot } = 0.77 \pm 0.07 , R _ { 1 } / R _ { \odot } = 0.012 \pm 0.001 , M _ { 2 } / M _ { \odot } = 0.30 \pm 0.10 , R _ { 2 } / R _ { \odot } = 0.34 \pm 0.04 , q = 0.39 \pm 0.12 , i = 82 ^ { \circ } \pm 4 ^ { \circ } and a / R _ { \odot } = 1.14 \pm 0.06 .

We have also obtained time-resolved I - and K -band photometry of DW UMa during the same low state .
Using Bessell ’ s spectral-type vs ( I - K ) color calibration , we estimate the spectral type of the donor star to be M 3.5 \pm 1.0 .
This latter result helps us to estimate the distance towards the system via Bailey ’ s method as d = 930 \pm 160 pc .

Finally , we have repeated Knigge et al. ’ s WD model atmosphere fit to the low-state UV spectrum of DW UMa in order to account for the higher surface gravity indicated by our eclipse analysis .
The best-fit model with surface gravity fixed at \log { g } = 8 has an effective temperature of T _ { eff } = 50 , 000 \pm 1000 K. The normalization of the fit also yields a second distance estimate , d = 590 \pm 100 pc .
If we adopt this distance and assume that the mid-eclipse K -band flux is entirely due to the donor star , we obtain a second estimate for the spectral type of the secondary in DW UMa , M 7 \pm 2.0 .
After discussing potential sources of systematic errors in both methods , we conclude that the true value for the distance and spectral type will probably be in between the values obtained by the two methods .